THYROID FUNCTION TESTS

The key tests to determine thyroid function are serum measurements of free thyroid hormones and thyroid-stimulating hormone (TSH). Thyroid hormones have a negative feedback on TSH secretion from the anterior pituitary. In hyperthyroidism, free thyroid hormones are increased above the normal range and TSH levels are markedly decreased. In hypothyroidism, free thyroid hormones are decreased and TSH concentrations are increased when the cause is disease of the thyroid gland; when caused by a deficiency of TSH, free thyroid hormones are decreased but TSH is usually low. Radioactive iodine studies of the thyroid gland, which used to be the mainstay of testing, have been supplanted by these blood tests.

Martin I. Surks

(see also: Goiter; Hyperthyroidism; Hypothyroidism; Iodine; Thyroid Disorders )

Bibliography

Kaptein, E. M., and Nelson, J. C. (1999). "Serum Thyroid Hormones and Thyroid-Stimulating Hormone." In Atlas of Clinical Endocrinology, Vol. I: Thyroid Diseases, ed. M. I. Surks. Philadelphia, PA: Current Medicine.