Laboratory tests for thyroid disorders.

The evolution in thyroid function tests since the 1960s has improved the physician's proficiency in accurately identifying thyroid dysfunction in a patient with suspected thyroid disease. The mainstays of modern thyroid testing strategies are the serum TSH concentration and AMA (anti-TPO) titer. The TSH level serves as an endogenous indicator of the biologically active free T4 fraction and, as a result, is currently the best gauge of the thyroid status of an individual. In addition, the TSH level has other advantages over free T4 estimates in confirming the presence of thyroid disease. First, each individual has his or her own free T4-TSH setpoint, whereby any deviation from this genetically determined relationship changes the serum TSH level. Second, owing to the local nature of the T4 feedback at the pituitary, these alterations in serum TSH values amplify small changes in circulating free T4 values. The net result of these unique attributes of measuring TSH is the ability to detect thyroid dysfunction early in the course of thyroid disease. There are limitations to the use of a TSH determination as a single thyroid function test, however. They include the presence of hypothalamic or pituitary disease or concurrent nonthyroidal illness and the immediate treatment of either hyperthyroidism or hypothyroidism. Because the majority of thyroid diseases involves autoimmune processes of the thyroid gland, the inclusion of an AMA titer in any approach to thyroid testing enhances both the diagnostic and prognostic expertise of the physician. In conclusion, currently available thyroid function tests have enhanced the diagnostic skills of the physician, but their effectiveness relies on clinical judgment rather than guidance from protocol or random application.