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Thyroid function tests
Thyroid function tests are commonly done to diagnose hyper- and hypo-thyroidism. Older tests such as basal metabolic rate, serum cholesterol, and radioactive iodine uptake are non-specific and have been replaced by more sensitive assays of serum T3, T4, FT3, FT4 and TSH. Measurement of thyroperoxidase antibodies is also useful to diagnose autoimmune causes of hypothyroidism like Hashimoto's thyroiditis. The TRH stimulation test can help distinguish between thyroid disorders of hypothalamic or pituitary origin.
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Thyroid function tests
- 1. Thyroid Function Tests R. C.Gupta M.D. (Biochemistry) Jaipur (Rajasthan), India
- 2. Thyroid function testsare commonly done to diagnose hyper- and hypo-thyroidism Most patients with thyroid disorders have only isolated signs and symptoms A clinical diagnosis is usually difficult Diagnosis often requires laboratory tests
- 3. Dietary iodide isabsorbed by the alimentary tract, and released into circulation Some of it is excreted in urine Some is taken up by the thyroid gland, and is converted into iodine Production of thyroid hormones
- 4. Iodine reacts withthe tyrosine residues of thyroglobulin Tyrosine is converted into mono-iodo- tyrosine (MIT) and di-iodo-tyrosine (DIT) Two DIT residues react with each other to form tetra-iodo-thyronine (thyroxine or T4) One residue each of MIT and DIT react to form tri-iodo-thyronine (T3)
- 5. T3 and T4are stored in thyroid acini, attached to thyroglobulin The hormones are released into blood as and when needed They are transported in circulation by thyroxine-binding globulin (TBG)
- 6. The TBG-bound hormonesare inactive Small amounts of T3 and T4 are released from TBG These exist as free T3 and T4 in circulation
- 7. Free T3 andT4 are the biologically active forms of the hormones T4 is more abundant than T3 but T3 is more powerful
- 8. Synthesis and releaseof T3 and T4 are increased by thyroid-stimulating hormone (TSH) of anterior pituitary TSH secretion is increased by thyrotropin- releasing hormone (TRH) of hypothalamus High blood levels of T3 and T4 cause feed- back inhibition of TSH and TRH secretion
- 10. Before the adventof radio-immuno- assay for hormones, tests for thyroid function were indirect and non-specific Some of the older tests are: • Basal metabolic rate • Serum cholesterol • Serum protein-bound iodine • Radioactive iodine uptake
- 11. Thyroid hormones stimulateall metabolic activities This increases the basal metabolic rate (BMR) Hence, BMR is raised in hyperthyroidism and lowered in hypothyroidism Basal metabolic rate
- 12. However, BMR isaffected by a number of physiological and pathological conditions Therefore, this test lacks specificity
- 13. Serum cholesterol levelis decreased in hyperthyroidism and increased in hypo- thyroidism But this test is non-specific because serum cholesterol can become abnormal in a number of other diseases Serum cholesterol
- 14. In this test,the iodine bound to serum proteins is measured Most of the protein-bound iodine (PBI) is present in T3 and T4 Serum protein-bound iodine
- 15. Serum PBI isincreased in hyperthyroidism and decreased in hypothyroidism However, accuracy of PBI measurement is poor
- 16. This test measuresthe uptake of circulating iodide by the thyroid gland A small dose of a radioactive isotope of iodine (5 µc of Na131I) is given to the subject orally in a fasting condition Radioactive iodine uptake
- 17. The radioactivity overthe thyroid gland is measured by a Geiger Muller counter after 2, 24 and 48 hours This is a measure of the radioactive iodine present in the thyroid gland
- 18. The normal uptakeof the ingested dose is: ▪ 1.5-15% at two hours ▪ 25-50% at 24 hours ▪ 15-40 % at 48 hours Radio-active iodine uptake is increased in hyperthyroidism and is decreased in hypo- thyroidism
- 19. Sensitive and specificmethods of hormone assays are available now Hence, the older tests have now become obsolete
- 20. Thyroid function testscommonly done now are: • Serum T3 and T4 • Serum Free T3 (FT3) and Free T4 (FT4) • Serum TSH • Thyroperoxidase antibodies (TPO Ab) • TRH stimulation test
- 21. Normal range oftotal T4 in serum is 5-12 µg/dl It is increased in hyperthyroidism and is decreased in hypothyroidism Serum T4
- 22. The normal rangeof total T3 in serum is 0.1-0.2 µg/dl It is increased in hyperthyroidism and is decreased in hypothyroidism An increase in serum T3 with normal T4 is seen in T3 toxicosis Serum T3
- 23. Sometimes, serum totalT4 may be normal despite the presence of a thyroid disorder In such cases, measurement of serum FT4 can clinch the diagnosis Normal range of serum FT4 is 0.8-2 ng/dl It is increased in hyperthyroidism and is decreased in hypothyroidism Serum Free T4 (FT4)
- 24. Like serum FT4,serum FT3 may be very useful in the diagnosis of subtle disorders of thyroid gland Normal range of serum FT3 is 0.3-0.6 ng/dl Serum Free T3 (FT3)
- 25. Serum TSH candifferentiate between primary thyroid disorders and those secondary to pituitary/hypothalamic disease In primary hyperthyroidism, serum TSH is decreased due to feedback inhibition In hyperthyroidism secondary to pituitary or hypothalamic disease, serum TSH is raised Serum TSH
- 26. In primary hypothyroidism,serum TSH is high due to lack of feedback inhibition In hypothyroidism secondary to pituitary or hypothalamic disease, it is decreased The normal range of serum TSH is 0.5-5 µU/ml
- 27. Hypothyroidism can sometimesoccur due to Hashimoto’s thyroiditis or post-partum thyroiditis In such cases, the disease results from an autoimmune response against thyro- peroxidase Thyroperoxidase antibodies (TPO Ab)
- 28. The autoimmune disordercan be diagnosed by detection of thyro- peroxidase antibodies in serum Thyroperoxidase antibodies are present in serum in Hashimoto’s thyroiditis and post-partum thyroiditis
- 29. Anterior pituitary isstimulated by TRH administration Serum T3, T4 and TSH are measured after stimulation This can distinguish between thyroid disorders of hypothalamic origin and pituitary origin TRH stimulation test
- 30. TRH stimulation testis rarely required in hyperthyroidism It is useful in hypothyroidism with low T3, low T4 and low TSH In such cases, a response to TRH stimulation points to hypothalamic origin of hypothyroidism
- 31. Subclinical hypothyroidism usuallyresults from autoimmune thyroiditis The patients have vague symptoms, normal serum T4 and high serum TSH These patients can progress to frank hypothyroidism later on Subclinical hypothyroidism
- 32. It is recommendedthat these patients should be screened for TPO antibodies If TPO antibodies are present, treatment is indicated If TPO antibodies are absent, annual follow up is recommended