Rational use of thyroid function tests.

In this era of cost containment, it is necessary to efficiently select the most important laboratory investigations for diagnostic and management purposes, with patient benefit as the ultimate objective. Thyroid function tests collectively represent a very costly item for laboratory services and have tended to be unselectively overutilized virtually until the present. Tests available for thyroid function testing include both in vitro and in vivo tests. Virtually all physicians are familiar with the free thyroxine (fT4) or its equivalents (fT4E), total and free triiodothyronine (TT3 and fT3), and sensitive thyrotrophin (TSH) assays, to which may be added plasma thyroglobulin (Tg), and thyroid autoantibodies (TAb) (including thyroid stimulating antibody [TSAb]). In vitro tests include thyroidal uptakes and scans, as well as other imaging techniques (e.g., ultrasound). Other less commonly used or obsolete techniques are not discussed. For screening or case finding where there is little probability that the patient(s) has thyroid dysfunction, a sensitive TSH assay is all that is initially required. If, however, the TSH result is elevated, a fT4E and TAb should then be performed. If the TSH is subnormal, then a fT4E, TT3, and (if still necessary) TSH response to TRH would determine if that patient was truly hyperthyroid (in elderly patients, a low TSH is often not associated with hyperthyroidism!). Conversely, not all patients with elevated fT4 are truly hyperthyroid. Uptakes and scans are primarily of importance in the correct diagnosis of unusual cases of hyperthyroidism and for nodular disease. When patients already diagnosed are being followed, proper selection of testing is again important, sometimes emphasizing one test over others (e.g., TT3); for example, in patients with Graves' Disease on anti-thyroid drugs, the fT4E and TSH values may not reflect the true status of the patient. On the one hand, the TSH may remain low for months after the patient has become euthyroid, and on the other hand, the fT4E may drop even below normal, whereas the T3 remains elevated (and the patient still hyperthyroid). The many other vagaries of these tests are also mentioned.