Total serum thyroxine includes both free and protein-bound thyroxine and is usually measured by competitive immunoassay. Normal level in adults is 5.0-12.0 μg/dl.
 
Test for total thyroxine or free thyroxine is usually combined with TSH measurement and together they give the best assessment of thyroid function.
 
Causes of Increased Total T4
 
  1. Hyperthyroidism: Elevation of both T4 and T3 values along with decrease of TSH are indicative of primary hyperthyroidism.
  2. Increased thyroxine-binding globulin: If concentration of TBG increases, free hormone level falls, release of TSH from pituitary is stimulated, and free hormone concentration is restored to normal. Reverse occurs if concentration of binding proteins falls. In either case, level of free hormones remains normal, while concentration of total hormone is altered. Therefore, estimation of only total T4 concentration can cause misinterpretation of results in situations that alter concentration of TBG.
  3. Factitious hyperthyroidism
  4. Pituitary TSH-secreting tumor.
 
Causes of Decreased Total T4
 
  1. Primary hypothyroidism: The combination of decreased T4 and elevated TSH are indicative of primary hypothyroidism.
  2. Secondary or pituitary hypothyroidism
  3. Tertiary or hypothalamic hypothyroidism
  4. Hypoproteinaemia, e.g. nephrotic syndrome
  5. Drugs: oestrogen, danazol
  6. Severe non-thyroidal illness.
 
Free Thyroxine (FT4)
 
FT4 comprises of only a small fraction of total T4, is unbound to proteins, and is the metabolically active form of the hormone. It constitutes about 0.05% of total T4. Normal range is 0.7 to 1.9 ng/dl. Free hormone concentrations (FT4 and FT3) correlate better with metabolic state than total hormone levels (since they are not affected by changes in TBG concentrations).
 
Measurement of FT4 is helpful in those situations in which total T4 level is likely to be altered due to alteration in TBG level (e.g. pregnancy, oral contraceptives, nephrotic syndrome).
 
Total and Free Triiodothyronine (T3)
 
Uses
 
  1. Diagnosis of T3 thyrotoxicosis: Hyperthyroidism with low TSH and elevated T3, and normal T4/FT4 is termed T3 thyrotoxicosis.
  2. Early diagnosis of hyperthyroidism: In early stage of hyperthyroidism, total T4 and free T4 levels are normal, but T3 is elevated.
 
A low T3 level is not useful for diagnosis of hypothyroidism since it is observed in about 25% of normal individuals.
 
For routine assessment of thyroid function, TSH and T4 are measured. T3 is not routinely estimated because normal plasma levels are very low.
 
Normal T3 level is 80-180 ng/dl.
 
Free T3: Measurement of free T3 gives true values in patients with altered serum protein levels (like pregnancy, intake of estrogens or oral contraceptives, and nephrotic syndrome). It represents 0.5% of total T3.
 
Thyrotropin Releasing Hormone (TRH) Stimulation Test
 
Uses
 
  1. Confirmation of diagnosis of secondary hypothyroidism
  2. Evaluation of suspected hypothalamic disease
  3. Suspected hyperthyroidism
 
This test is not much used nowadays due to the availability of sensitive TSH assays.
 
Procedure
 
  • A baseline blood sample is collected for estimation of basal serum TSH level.
  • TRH is injected intravenously (200 or 500 μg) followed by measurement of serum TSH at 20 and 60 minutes.
 
Interpretation
 
  1. Normal response: A rise of TSH > 2 mU/L at 20 minutes, and a small decline at 60 minutes.
  2. Exaggerated response: A further significant rise in already elevated TSH level at 20 minutes followed by a slight decrease at 60 minutes; occurs in primary hypothyroidism.
  3. Flat response: There is no response; occurs in secondary (pituitary) hypothyroidism.
  4. Delayed response: TSH is higher at 60 minutes as compared to its level at 20 minutes; seen in tertiary (hypothalamic) hypothyroidism.
 
Antithyroid Antibodies
 
Box 864.1 Thyroid autoantibodies
 
  • Useful for diagnosis and monitoring of autoimmune thyroid diseases.
  • Antimicrosomal or antithyroid peroxidase antibodies: Hashimoto’s thyroiditis
  • Anti-TSH receptor antibodies: Graves’ disease
Various autoantibodies (TSH receptor, antimicrosomal, and antithyroglobulin) are detected in thyroid disorders like Hashimoto’s thyroiditis and Graves’ disease. Antimicrosomal (also called as thyroid peroxidase) and anti-thyroglobulin antibodies are observed in almost all patients with Hashimoto’s disease. TSH receptor antibodies (TRAb) are mainly tested in Graves’ disease to predict the outcome after treatment (Box 864.1).
 
Radioactive Iodine Uptake (RAIU) Test
 
This is a direct test that assesses the trapping of iodide by thyroid gland (through the iodine symporters or pumps in follicular cells) for thyroid hormone synthesis. Patient is administered a tracer dose of radioactive iodine (131I or 123I) orally. This is followed by measurement of amount of radioactivity over the thyroid gland at 2 to 6 hours and again at 24 hours. RAIU correlates directly with the functional activity of the thyroid gland. Normal RAIU is about 10-30% of administered dose at 24 hours, but varies according to the geographic location due to differences in dietary intake.
 
Causes of Increased Uptake
 
  • Hyperthyroidism due to Graves’ disease, toxic multinodular goiter, toxic adenoma, TSH-secreting tumor.
 
Causes of Decreased Uptake
 
  • Hyperthyroidism due to administration of thyroid hormone, factitious hyperthyroidism, subacute thyroiditis.
 
Uses
 
RAIU is most helpful in differential diagnosis of hyperthyroidism by separating causes into those due to increased uptake and due to decreased uptake.
 
Thyroid Scintiscanning
 
An isotope (99mTc-pertechnetate) is administered and a gamma counter assesses its distribution within the thyroid gland.
 
Interpretation
 
  • Differential diagnosis of high RAIU thyrotoxicosis:
    – Graves’ disease: Uniform or diffuse increase in uptake
    – Toxic multinodular goiter: Multiple discrete areas of increased uptake
    – Adenoma: Single area of increased uptake
  • Evaluation of a solitary thyroid nodule:
    – ‘Hot’ nodule: Hyperfunctioning
    – ‘Cold’ nodule: Non-functioning; about 20% cases are malignant.
 
Interpretation of thyroid function tests is shown in Table 164.1.
 
Table 864.1 Interpretation of thyroid function tests
Test results Interpretations
1. TSH Normal, FT4 Normal Euthyroid
2. Low TSH, Low FT4 Secondary hypothyroidism
3. High TSH, Normal FT4 Subclinical hypothyroidism
4. High TSH, Low FT4 Primary hypothyroidism
5. Low TSH, Normal FT4, Normal FT3 Subclinical hyperthyroidism
6. Low TSH, Normal FT4, High FT3 T3 toxicosis
7. Low TSH, High FT4 Primary hyperthyroidism
 
Neonatal Screening for Hypothyroidism
 
Thyroid hormone deficiency during neonatal period can cause severe mental retardation (cretinism) that can be prevented by early detection and treatment. Estimation of TSH is done on dry blood spots on filter paper or cord serum between 3rd to 5th days of life. Elevated TSH is diagnostic of hypothyroidism. In infants with confirmed hypothyroidism, RAIU (123I) scan should be done to distinguish between thyroid agenesis and dyshormonogenesis.

Among the endocrine disorders, disorders of the thyroid are common and are only next in frequency to diabetes mellitus. They are more common in women than in men. Functional thyroid disorders can be divided into two types depending on the activity of the thyroid gland: hypothyroidism (low thyroid hormones), and hyperthyroidism (excess thyroid hormones).

The ovaries are the sites of production of female gametes or ova by the process of oogenesis. The ova are released by the process of ovulation in a cyclical manner at regular intervals. Ovary contains numerous follicles that contain ova in various stages of development. During each menstrual cycle, up to 20 primordial follicles are activated for maturation; however, only one follicle becomes fully mature; this dominant follicle ruptures to release the secondary oocyte from the ovary. Maturation of the follicle is stimulated by follicle stimulating hormone (FSH) secreted by anterior pituitary (Figure 862.1). Maturing follicle secretes estrogen that causes proliferation of endometrium of the uterus (proliferative phase). Follicular cells also secrete inhibin which regulates release of FSH by the anterior pituitary. Fall in FSH level is followed by secretion of luteinizing hormone (LH) by the anterior pituitary (LH surge). This causes follicle to rupture and the ovum is expelled into the peritoneal cavity near the fimbrial end of the fallopian tube. The fallopian tubes conduct ova from the ovaries to the uterus. Fertilization of ovum by the sperm occurs in the fallopian tube.
 
Figure 862.1 The hypothalamus pituitary ovarian axis
Figure 862.1 The hypothalamus-pituitary-ovarian axis 
 
The ovum consists of the secondary oocyte, zona pellucida and corona radiata. The ruptured follicle in the ovary collapses and fills with blood clot (corpus luteum). LH converts granulose cells in the follicle to lutein cells which begin to secrete progesterone. Progesterone stimulates secretion from the endometrial glands (secretory phase) that were earlier under the influence of estrogen. Rising progesterone levels inhibit LH production from the anterior pituitary. Without LH, the corpus luteum regresses and becomes functionless corpus albicans. After regression of corpus luteum, production of estrogen and progesterone stops and endometrium collapses, causing onset of menstruation. If the ovum is fertilized and implanted in the uterine wall, human chorionic gonadotropin (hCG) is secreted by the developing placenta into the maternal circulation. Human chorionic gonadotropin maintains the corpus luteum for secetion of estrogen and progesterone till 12th week of pregnancy. After 12th week, corpus luteum regresses to corpus albicans and the function of synthesis of estrogen and progesterone is taken over by placenta till parturition.
 
The average duration of the normal menstrual cycle is 28 days. Ovulation occurs around 14th day of the cycle. The time interval between ovulation and menstruation is called as luteal phase and is fairly constant (14 days) (Figure 862.2).
 
Figure 862.2 Normal menstrual cycle
Figure 862.2 Normal menstrual cycle
 
Causes of Female Infertility
 
Causes of female infertility are shown in Table 862.1.
 
Table 862.1 Causes of female infertility
1. Hypothalamic-pituitary dysfunction:
  • Hypothalamic causes
    – Excessive exercise
    – Excess stress
    – Low weight
    – Kallman’s syndrome
    Idiopathic
  • Pituitary causes
    – Hyperprolactinemia
    Hypopituitarism (Sheehan’s syndrome, Simmond’s disease)
    – Craniopharyngioma
    – Cerebral irradiation
 2. Ovarian dysfunction:
  • Polycystic ovarian disease (Stein-Leventhal syndrome)
  • Luteinized unruptured follicle
  • Turner’s syndrome
  • Radiation or chemotherapy
  • Surgical removal of ovaries
  • Idiopathic
 3. Dysfunction in passages:
  • Fallopian tubes
    Infections: Tuberculosis, gonorrhea, Chlamydia
    – Previous surgery (e.g. laparotomy)
    – Tubectomy
    Congenital hypoplasia, non-canalization
    Endometriosis
  • Uterus
    – Uterine malformations
    – Asherman’s syndrome
    – Tuberculous endometritis
    Fibroid
  • Cervix: Sperm antibodies
  • Vagina: Septum
 4. Dysfunction of sexual act: Dyspareunia
 
Investigations
 
Evaluation of female infertility is shown in Figure 862.3.
 
Figure 862.3 Evaluation of female infertility
Figure 862.3 Evaluation of female infertility. FSH: Follicle stimulating hormone; LH: Luteinizing hormone; DHEA-S: Dihydroepiandrosterone; TSH: Thyroid stimulating hormone; ↑ : Increased; ↓ : Decreased
 
Tests for Ovulation
 
Most common cause of female infertility is anovulation.
 
  1. Regular cycles, mastalgia, and laparoscopic direct visualization of corpus luteum indicate ovulatory cycles. Anovulatory cycles are clinically characterized by amenorrhea, oligomenorrhea, or irregular menstruation. However, apparently regular cycles may be associated with anovulation.
  2. Endometrial biopsy: Endometrial biopsy is done during premenstrual period (21st-23rd day of the cycle). The secretory endometrium during the later half of the cycle is an evidence of ovulation.
  3. Ultrasonography (USG): Serial ultrasonography is done from 10th day of the cycle and the size of the dominant follicle is measured. Size >18 mm is indicative of imminent ovulation. Collapse of the follicle with presence of few ml of fluid in the pouch of Douglas is suggestive of ovulation. USG also is helpful for treatment (i.e. timing of coitus or of intrauterine insemination) and diagnosis of luteinized unruptured follicle (absence of collapse of dominant follicle). Transvaginal USG is more sensitive than abdominal USG.
  4. Basal body temperature (BBT): Patient takes her oral temperature at the same time every morning before arising. BBT falls by about 0.5°F at the time of ovulation. During the second (progestational) half of the cycle, temperature is slightly raised above the preovulatory level (rise of 0.5° to 1°F). This is due to the slight pyrogenic action of progesterone and is therefore presumptive evidence of functional corpus luteum.
  5. Cervical mucus study:
    Fern test: During estrogenic phase, a characteristic pattern of fern formation is seen when cervical mucus is spread on a glass slide (Figure 862.4). This ferning disappears after the 21st day of the cycle. If previously observed, its disappearance is presumptive evidence of corpus luteum activity.
    Spinnbarkeit test: Cervical mucus is elastic and withstands stretching upto a distance of over 10 cm. This phenomenon is called Spinnbarkeit or the thread test for the estrogen activity. During the secretory phase, viscosity of the cervical mucus increases and it gets fractured when stretched. This change in cervical mucus is evidence of ovulation.
  6. Vaginal cytology: Karyopyknotic index (KI) is high during estrogenic phase, while it becomes low in secretory phase. This refers to percentage of super-ficial squamous cells with pyknotic nuclei to all mature squamous cells in a lateral vaginal wall smear. Usually minimum of 300 cells are evaluated. The peak KI usually corresponds with time of ovulation and may reach upto 50 to 85.
  7. Estimation of progesterone in mid-luteal phase (day 21 or 7 days before expected menstruation): Progesterone level > 10 nmol/L is a reliable evidence of ovulation if cycles are regular (Figure 862.5). A mistimed sample is a common cause of abnormal result.
 
Figure 862.4 Ferning of cervical mucosa
Figure 862.4 Ferning of cervical mucosa
 
Figure 862.5 Serum progesterone during normal menstrual cycle
Figure 862.5 Serum progesterone during normal menstrual cycle
 
Tests to Determine the Cause of Anovulation
 
  1. Measurement of LH, FSH, and estradiol during days 2 to 6: All values are low in hypogonadotropic hypogonadism (hypothalamic or pituitary failure).
  2. Measurement of TSH, prolactin, and testosterone if cycles are irregular or absent:
    Increased TSH: Hypothyroidism
    Increased prolactin: Pituitary adenoma
    Increased testosterone: Polycystic ovarian disease (PCOD), congenital adrenal hyperplasia (To differentiate PCOD from congenital adrenal hyperplasia, ultrasound and estimation of dihydroepiandrosterone or DHEA are done).
  3. Transvaginal ultrasonography: This is done for detection of PCOD.
 
Investigations to Assess Tubal and Uterine Status
 
  1. Infectious disease: These tests include endometrial biopsy for tuberculosis and test for chlamydial IgG antibodies for tubal factor in infertility.
  2. Hysterosalpingography (HSG): HSG is a radiological contrast study for investigation of the shape of the uterine cavity and for blockage of fallopian tubes (Figure 862.6). A catheter is introduced into the cervical canal and a radiocontrast dye is injected into the uterine cavity. A real time X-ray imaging is carried out to observe the flow of the dye into the uterine cavity, tubes, and spillage into the uterine cavity.
  3. Hysterosalpingo-contrast sonography: A catheter is introduced into the cervical canal and an echocontrast fluid is introduced into the uterine cavity. Shape of the uterine cavity, filling of fallopian tubes, and spillage of contrast fluid are noted. In addition, ultrasound scan of the pelvis provides information about any fibroids or polycystic ovarian disease.
  4. Laparoscopy and dye hydrotubation test with hysteroscopy: In this test, a cannula is inserted into the cervix and methylene blue dye is introduced into the uterine cavity. If tubes are patent, spillage of the dye is observed from the ends of both tubes. This technique also allows visualization of pelvic organs, endometriosis, and pelvic adhesions. If required, endometriosis and tubal blockage can be treated during the procedure.
 
Possible pregnancy and active pelvic or vaginal infection are contraindications to tubal patency tests.
 
Figure 862.6 Hysterosalpingography
Figure 862.6 Hysterosalpingography
The male reproductive system consists of testes (paired organs located in the scrotal sac that produce spermatozoa and secrete testosterone), a paired system of ducts comprising of epididymis, vasa deferentia, and ejaculatory ducts (collect, store, and conduct spermatozoa), paired seminal vesicles and a single prostate gland (produce nutritive and lubricating seminal fluid), bulbourethral glands of Cowper (secrete lubricating mucus), and penis (organ of copulation).
 
The hypothalamus secretes gonadotropin releasing hormone (GnRH) that regulates the secretion of the two gonadotropins from the anterior pituitary: luteinizing hormone (LH) and follicle stimulating hormone (FSH) (Figure 861.1). Luteinizing hormone primarily stimulates the production and secretion of testosterone from Leydig cells located in the interstitial tissue of the testes. Testosterone stimulates spermatogenesis, and plays a role in the development of secondary sexual characters. Testosterone needs to be converted to an important steroidal metabolite, dihydrotestosterone within cells to perform most of its androgenic functions. Testosterone inhibits LH secretion by negative feedback. Follicle stimulating hormone acts on Sertoli cells of seminiferous tubules to regulate the normal maturation of the sperms. Sertoli cells produce inhibin that controls FSH secretion by negative feedback.
 
Figure 861.1 Hypothalamus-pituitary-testis axis. + indicates stimulation; – indicates negative feedback
Figure 861.1 Hypothalamus-pituitary-testis axis. + indicates stimulation; – indicates negative feedback
 
During sexual intercourse, semen is deposited into the vagina. Liquefaction of semen occurs within 20-30 minutes due to proteolytic enzymes of prostatic fluid. For fertilization to occur in vivo, the sperm must undergo capacitation and acrosome reaction. Capacitation refers to physiologic changes in sperms that occur during their passage through the cervix of the female genital tract. With capacitation, the sperm acquires (i) ability to undergo acrosome reaction, (ii) ability to bind to zona pellucida, and (iii) hypermotility. Sperm then travels through the cervix and uterus up to the fallopian tube. Binding of sperm to zona pellucida induces acrosomal reaction (breakdown of outer plasma membrane by enzymes of acrosome and its fusion with outer acrosomal membrane, i.e. loss of acrosome). This is necessary for fusion of sperm and oocyte membranes. Acrosomal reaction and binding of sperm and ovum surface proteins is followed by penetration of zona pellucida of ovum by the sperm. Following penetration by sperm, hardening of zona pellucida occurs that inhibits penetration by additional sperms. A sperm penetrates and fertilizes the egg in the ampullary portion of the fallopian tube (Figure 861.2).
 
Figure 861.2 Steps before and after fertilization of ovum
Figure 861.2 Steps before and after fertilization of ovum
 
Causes of Male Infertility
 
Causes of male infertility are listed in Table 861.1.
 
Table 861.1 Causes of male infertility 
2. Hypothalamic-pituitary dysfunction (hypogonadotropic hypogonadism)
3. Testicular dysfunction:
  • Radiation, cytotoxic drugs, antihypertensives, antidepressants
  • General factors like stress, emotional factors, drugs like marijuana, anabolic steroids, and cocaine, alcoholism, heavy smoking, undernutrition
  • Mumps orchitis after puberty
  • Varicocele (dilatation of pampiniform plexus of scrotal veins)
  • Undescended testes (cryptorchidism)
  • Endocrine disorders like diabetes mellitus, thyroid dysfunction
  • Genetic disorders: Klinefelter’s syndrome, microdeletions in Y chromosome, autosomal Robertsonian translocation, immotile cilia syndrome (Kartagener’s syndrome), cystic fibrosis, androgen receptor gene defect
4. Dysfunction of passages and accessory sex glands:
 5. Dysfunction of sexual act:
  • Defects in ejaculation: retrograde (semen is pumped backwards in to the bladder), premature, or absent
  • Hypospadias
 
Investigations of Male Infertility
 
  1. History: This includes type of lifestyle (heavy smoking, alcoholism), sexual practice, erectile dysfunction, ejaculation, sexually transmitted diseases, surgery in genital area, drugs, and any systemic illness.
  2. Physical examination: Examination of reproductive system should includes testicular size, undescended testes, hypospadias, scrotal abnormalities (like varicocele), body hair, and facial hair. Varicocele can occur bilaterally and is the most common surgically removable abnormality causing male infertility.
  3. Semen analysis: See article Semen Analysis. Evaluation of azoospermia is shown in Figure 861.3. Evaluation of low semen volume is shown in Figure 861.4.
  4. Chromosomal analysis: This can reveal Klinefelter’s syndrome (e.g. XXY karyotype) (Figure 861.5), deletion in Y chromosome, and autosomal Robertsonian translocation. It is necessary to screen for cystic fibrosis carrier state if bilateral congenital absence of vas deferens is present.
  5. Hormonal studies: This includes measurement of FSH, LH, and testosterone to detect hormonal abnormalities causing testicular failure (Table 861.2).
  6. Testicular biopsy: Testicular biopsy is indicated when differentiation between obstructive and non-obstructive azoospermia is not evident (i.e. normal FSH and normal testicular volume).
 
Table 861.2 Interpretation of hormonal studies in male infertility 
Follicle stimulating hormone Luteinizing hormone Testosterone Interpretation
Low Low Low Hypogonadotropic hypogonadism (Hypothalamic or pituitary disorder)
High High Low Hypergonadotropic hypogonadism (Testicular disorder)
Normal Normal Normal Obstruction of passages, dysfunction of accessory glands
 
Figure 861.3 Evaluation of azoospermia
Figure 861.3 Evaluation of azoospermia. FSH: Follicle stimulating hormone; LH: Luteinizing hormone
 
Figure 861.4 Evaluation of low semen volume
Figure 861.4 Evaluation of low semen volume
 
Figure 861.5 Karyotype in Klinefelter's Syndrome
 Figure 861.5 Karyotype in Klinefelter’s syndrome (47, XXY)
 
Common initial investigations for diagnosis of cause of infertility are listed below.
 

In Quick

Researchers have recognized a fungus which can damage down plastics. The species could be a beneficial device as we strive to lessen the impact of waste fabric at the surroundings.

Fungi feast

Researchers at the Chinese Academy of Sciences’ Kunming Institute of Botany have discovered a fungus that could doubtlessly assist us to deal with the trouble of non-biodegradable plastics. The fungus is ready to break down waste plastics in a depend on weeks that might in any other case persist in the environment for years.

Aspergillus tubingensis is commonly observed in soil, however, the have a look at observed that it is able to additionally thrive at the surface of plastics. It secretes enzymes which smash down the bonds between person molecules after which use its mycelia to break them aside.

It’s conceivable that there are all types of fungi with beneficial properties that we don’t but know approximately — but as deforestation and different human activity hold to spoil habitats, we would never advantage get admission to such species. The researchers actually found Aspergillus tubingensis on a rubbish unload in Islamabad, Pakistan.

Plastic Capability

The have a look at found that there are several elements that affect the fungus’ capability to interrupt down plastic. The temperature and pH stability of its environment, in addition to the kind of lifestyle medium in the vicinity, had an impact on its performance.

The following step for those researchers is to figure out what conditions might be ideal to help facilitate a realistic implementation.

The fungus might be used to assist address the problem of plastic particles swimming around in our water supply via being put to paintings in a waste remedy plant, or in soil contaminated with the cloth. The benefits of mycoremediation — the exercise of the usage of fungi to degrade undesirable substances — have become increasingly more apparent as we discover species which could degrade extra varieties of fabric.

Anatomically, stomach is divided into four parts: cardia, fundus, body, and pyloric part. Cardia is the upper part surrounding the entrance of the esophagus and is lined by the mucus-secreting epithelium. The epithelium of the fundus and the body of the stomach is composed of different cell types including: (i) mucus-secreting cells which protect gastric mucosa from self-digestion by forming an overlying thick layer of mucus, (ii) parietal cells which secrete hydrochloric acid and intrinsic factor, and (iii) peptic cells or chief cells which secrete the proteolytic enzyme pepsinogen. Pyloric part is divided into pyloric antrum and pyloric canal. It is lined by mucus-secreting cells and gastrin-secreting neuroendocrine cells (G cells) (Figure 859.1).
 
Figure 859.1 Parts of stomach and their lining cells
Figure 859.1 Parts of stomach and their lining cells 
 
In the stomach, ingested food is mechanically and chemically broken down to form semi-digested liquid called chyme. Following relaxation of pyloric sphincter, chyme passes into the duodenum.
 
There are three phases of gastric acid secretion: cephalic, gastric, and intestinal.
 
  • Cephalic or neurogenic phase: This phase is initiated by the sight, smell, taste, or thought of food that causes stimulation of vagal nuclei in the brain. Vagus nerve directly stimulates parietal cells to secrete acid; in addition, it also stimulates antral G cells to secrete gastrin in blood (which is also a potent stimulus for gastric acid secretion) (Figure 859.2). Cephalic phase is abolished by vagotomy.
  • Gastric phase: Entry of swallowed food into the stomach causes gastric distension and induces gastric phase. Distension of antrum and increase in pH due to neutralization of acid by food stimulate antral G cells to secrete gastrin into the circulation. Gastrin, in turn, causes release of hydrochloric acid from parietal cells.
  • Intestinal phase: Entry of digested proteins into the duodenum causes an increase in acid output from the stomach. It is thought that certain hormones and absorbed amino acids stimulate parietal cells to secrete acid.
 
The secretion from the stomach is called as gastric juice. The chief constituents of the gastric juice are:
 
  • Hydrochloric acid (HCl): This is secreted by the parietal cells of the fundus and the body of the stomach. HCl provides the high acidic pH necessary for activation of pepsinogen to pepsin. Gastric acid secretion is stimulated by histamine, acetylcholine, and gastrin (Figure 859.2). HCl kills most microorganisms entering the stomach and also denatures proteins (breaks hydrogen bonds making polypeptide chains to unfold). Its secretion is inhibited by somatostatin (secreted by D cells in pancreas and by mucosa of intestine), gastric inhibitory peptide (secreted by K cells in duodenum and jejunum), prostaglandin, and secretin (secreted by S cells in duodenum).
  • Pepsin: Pepsin is secreted by chief cells in stomach. Pepsin causes partial digestion of proteins leading to the formation of large polypeptide molecules (optimal function at pH 1.0 to 3.0). Its secretion is enhanced by vagal stimulation.
  • Mucus
  • Intrinsic factor (IF): IF is necessary for absorption of vitamin B12 in the terminal ileum. It is secreted by parietal cells of stomach.
 
Figure 859.2 Stimulation of gastric acid secretion
Figure 859.2 Stimulation of gastric acid secretion. Three receptors on parietal cells stimulate acid secretion: histamine (H2) receptor, acetylcholine or cholinergic receptor, and gastrin/CCK-B receptor. Histamine is released by enterochromaffin-like cells in lamina propria. Acetylcholine is released from nerve endings. Gastrin is released from G cells in antrum (in response to amino acids in food, antral distention, and gastrin-releasing peptide). After binding to receptors, H+ is secreted in exchange for K+ by proton pump
  • Gastric intubation for gastric analysis is contraindicated in esophageal stricture or varices, active nasopharyngeal disease, diverticula, malignancy, recent history of severe gastric hemorrhage, hypertension, aortic aneurysm, cardiac arrhythmias, congestive cardiac failure, or non-cooperative patient.
  • Pyloric stenosis: Obstruction of gastric outlet can elevate gastric acid output due to raised gastrin (following antral distension).
  • Pentagastrin stimulation is contraindicated in cases with allergy to pentagastrin, and recent severe gastric hemorrhge due to peptic ulcer disease.
 
Gastric analysis is not a commonly performed procedure because of following reasons:
 
  • It is an invasive and cumbersome technique that is traumatic and unpleasant for the patient.
  • Information obtained is not diagnostic in itself.
  • Availability of better tests for diagnosis such as endoscopy and radiology (for suspected peptic ulcer or malignancy); serum gastrin estimation (for ZE syndrome); vitamin assays, Schilling test, and antiparietal cell antibodies (for pernicious anemia); and tests for Helicobacter pylori infection (in duodenal or gastric ulcer).
  • Availability of better medical line of treatment that obviates need for surgery in many patients.

Hollander’s test (Insulin hypoglycemia test):

In the past, this test was used for confirmation of completeness of vagotomy (done for duodenal ulcer). Hypoglycemia is a potent stimulus for gastric acid secretion and is mediated by vagus nerve. This response is abolished by vagotomy.

In this test, after determining BAO, insulin is administered intravenously (0.15-0.2 units/kg) and acid output is estimated every 15 minutes for 2 hours (8 post-stimulation samples). Vagotomy is considered as complete if, after insulin-induced hypoglycemia (blood glucose < 45 mg/dl), no acid output is observed within 45 minutres.

The test gives reliable results only if blood glucose level falls below 50 mg/dl at some time following insulin injection. It is best carried out after 3-6 months of vagotomy.

The test is no longer recommended because of the risk associated with hypoglycemia. Myocardial infarction, shock, and death have also been reported.

Fractional test meal:

In the past, test meals (e.g. oat meal gruel, alcohol) were administered orally to stimulate gastric secretion and determine MAO or PAO. Currently, parenteral pentagastrin is the gastric stimulant of choice.

Tubeless gastric analysis:

This is an indirect and rapid method for determining output of free hydrochloric acid in gastric juice. In this test, a cationexchange resin tagged to a dye (azure A) is orally administered. In the stomach, the dye is displaced from the resin by the free hydrogen ions of the hydrochloric acid. The displaced azure A is absorbed in the small intestine, enters the bloodstream, and is excreted in urine. Urinary concentration of the dye is measured photometrically or by visual comparison with known color standards. The quantity of the dye excreted is proportional to the gastric acid output. However, if kidney or liver function is impaired, false results may be obtained. The test is no longer in use.

Spot check of gastric pH:

According to some investigators, spot determination of pH of fasting gastric juice (obtained by nasogastric intubation) can detect the presence of hypochlorhydria (if pH>5.0 in men or >7.0 in women).

Congo red test during esophagogastroduodenoscopy:

This test is done to determine the completeness of vagotomy. Congo red dye is sprayed into the stomach during esophagogastroduodenoscopy; if it turns red, it indicates presence of functional parietal cells in stomach with capacity of producing acid.

REFERENCE RANGES

  • Volume of gastric juice: 20-100 ml
  • Appearance: Clear
  • pH: 1.5 to 3.5
  • Basal acid output: Up to 5 mEq/hour
  • Peak acid output: 1 to 20 mEq/hour
  • Ratio of basal acid output to peak acid output: <0.20 or < 20%
In gastric analysis, amount of acid secreted by the stomach is determined on aspirated gastric juice sample. Gastric acid output is estimated before and after stimulation of parietal cells (i.e. basal and peak acid output). This test was introduced in the past mainly for the evaluation of peptic ulcer disease (to assess the need for operative intervention). However, diminishing frequency of peptic ulcer disease and availability of safe and effective medical treatment have markedly reduced the role of surgery.
 
  1. To determine the cause of recurrent peptic ulcer disease:
    To detect Zollinger-Ellison (ZE) syndrome: ZE syndrome is a rare disorder in which multiple mucosal ulcers develop in the stomach, duodenum, and upper jejunum due to gross hypersecretion of acid in the stomach. The cause of excess secretion of acid is a gastrin-producing tumor of pancreas. Gastric analysis is done to detect markedly increased basal and pentagastrinstimulated gastric acid output for diagnosis of ZE syndrome (and also to determine response to acidsuppressant therapy). However, a more sensitive and specific test for diagnosis of ZE syndrome is measurement of serum gastrin (fasting and secretin-stimulated).
    To decide about completeness of vagotomy following surgery for peptic ulcer disease: See Hollander’s test.
  2. To determine the cause of raised fasting serum gastrin level: Hypergastrinemia can occur in achlorhydria, Zollinger-Ellison syndrome, and antral G cell hyperplasia.
  3. To support the diagnosis of pernicious anemia (PA): Pernicious anemia is caused by defective absorption of vitamin B12 due to failure of synthesis of intrinsic factor secondary to gastric mucosal atrophy. There is also absence of hydrochloric acid in the gastric juice (achlorhydria). Gastric analysis is done for demonstration of achlorhydria if facilities for vitamin assays and Schilling’s test are not available (Achlorhydria by itself is insufficient for diagnosis of PA).
  4. To distinguish between benign and malignant ulcer: Hypersecretion of acid is a feature of duodenal peptic ulcer, while failure of acid secretion (achlorhydria) occurs in gastric carcinoma. However, anacidity occurs only in a small proportion of cases with advanced gastric cancer. Also, not all patients with duodenal ulcer show increased acid output.
  5. To measure the amount of acid secreted in a patient with symptoms of peptic ulcer dyspepsia but normal X-ray findings: Excess acid secretion in such cases is indicative of duodenal ulcer. However, hypersecretion of acid does not always occur in duodenal ulcer.
  6. To decide the type of surgery to be performed in a patient with peptic ulcer: Raised basal as well as peak acid outputs indicate increased parietal cell mass and need for gastrectomy. Raised basal acid output with normal peak output is an indication for vagotomy.

To assess gastric acid secretion, acid output from the stomach is measured in a fasting state and after injection of a drug which stimulates gastric acid secretion. Basal acid output (BAO) is the amount of hydrochloric acid (HCl) secreted in the absence of any external stimuli (visual, olfactory, or auditory). Maximum acid output (MAO) is the amount of hydrochloric acid secreted by the stomach following stimulation by pentagastrin. MAO is calculated from the first four 15-minute samples after stimulation. Peak acid output (PAO) is calculated from the two highest consecutive 15-minute samples. It indicates greatest possible acid secretory capacity and is preferred over MAO as it is more reproducible. Acidity is estimated by titration.

Bioethics is the study of the ethical issues emerging from advances in biology and medicine. It is also moral discernment as it relates to medical policy and practice. Bioethicists are concerned with the ethical questions that arise in the relationships among life sciences, biotechnology, medicine, politics, law, and philosophy. It includes the study of values ("the ethics of the ordinary") relating to primary care and other branches of medicine.
Animal biotechnology is a branch of biotechnology in which molecular biology techniques are used to genetically engineer (i.e. modify the genome of) animals in order to improve their suitability for pharmaceutical, agricultural or industrial applications. Animal biotechnology has been used to produce genetically modified animals that synthesize therapeutic proteins, have improved growth rates or are resistant to disease.
Biophysics or biological physics is an interdisciplinary science that applies the approaches and methods of physics to study biological systems. Biophysics covers all scales of biological organization, from molecular to organismic and populations. Biophysical research shares significant overlap with biochemistry, physical chemistry, nanotechnology, bioengineering, computational biology, biomechanics and systems biology.
 
The term biophysics was originally introduced by Karl Pearson in 1892.
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