- Hyperthyroidism: Elevation of both T4 and T3 values along with decrease of TSH are indicative of primary hyperthyroidism.
- 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.
- Factitious hyperthyroidism
- Pituitary TSH-secreting tumor.
- Primary hypothyroidism: The combination of decreased T4 and elevated TSH are indicative of primary hypothyroidism.
- Secondary or pituitary hypothyroidism
- Tertiary or hypothalamic hypothyroidism
- Hypoproteinaemia, e.g. nephrotic syndrome
- Drugs: oestrogen, danazol
- Severe non-thyroidal illness.
- Diagnosis of T3 thyrotoxicosis: Hyperthyroidism with low TSH and elevated T3, and normal T4/FT4 is termed T3 thyrotoxicosis.
- Early diagnosis of hyperthyroidism: In early stage of hyperthyroidism, total T4 and free T4 levels are normal, but T3 is elevated.
- Confirmation of diagnosis of secondary hypothyroidism
- Evaluation of suspected hypothalamic disease
- Suspected hyperthyroidism
- 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.
- Normal response: A rise of TSH > 2 mU/L at 20 minutes, and a small decline at 60 minutes.
- 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.
- Flat response: There is no response; occurs in secondary (pituitary) hypothyroidism.
- Delayed response: TSH is higher at 60 minutes as compared to its level at 20 minutes; seen in tertiary (hypothalamic) hypothyroidism.
Box 864.1 Thyroid autoantibodies
- Hyperthyroidism due to Graves’ disease, toxic multinodular goiter, toxic adenoma, TSH-secreting tumor.
- Hyperthyroidism due to administration of thyroid hormone, factitious hyperthyroidism, subacute thyroiditis.
- 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.
|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|