Serum thyroxine (T₄) concentration was, for many years, the most useful first-line test of thyroid function. Historically, T₄ was estimated by measuring the serum protein bound iodine or the butanol extractable iodine; these measures are no longer used. (In suspected iodide ingestion, the protein bound iodine is much greater than the T₄.) Serum T₄ is determined almost exclusively by RIA; values in euthyroid patients range between 5 and 12 μg/dL. In hyperthyroidism, higher levels are present, and in hypothyroidism, lower levels are seen.

In general, the level of serum triiodothyronine (T₃), the most active thyroid hormone, varies in parallel with the T₄ level. Healthy subjects have values of 80 to 200 ng/dL, with higher levels occurring in hyperthyroidism. Indeed, the T₃ levels may increase despite a normal serum T₄ level, a condition known as T₃ toxicosis. Whereas T₄ is usually low in hypothyroidism, serum T₃ is often in the normal range, possibly owing to increased conversion of T₄ to T₃. Also, the peripheral conversion of T₄ to T₃ is exquisitely sensitive to acute stresses of many causes. Thus, a low serum T₃ level is to be expected in many nonthyroidal illnesses3, whereas serum T₄ levels may be low, normal, or elevated.

Serum reverse T₃ (rT₃) ranges from 10 to 60 ng/dL in euthyroid patients. Like T₄, serum rT₃ level is high in hyperthyroidism and low in hypothyroidism. In nonthyroidal illness, serum rT₃ concentrations are often elevated,4 whereas T₄ levels are normal or low, and T₃ levels are reduced. The discrepant results for T₃ and rT₃ in such disorders have made rT₃ measurements clinically useful in differentiating primary hypothyroidism from acute illness.

Although RIAs have been developed for the diiodothyronines,4 monoiodothyronines,5,6 measurement of and T₄ and T₃ sulfates7,8, these iodothyronine metabolites has no current clinical utility.