Thyroid hormone drugs affecting binding

Drugs can affect thyroid function in a number of ways.41 Effects of drugs on thyroid hormone protein binding, LT4 absorption, and metabolism have been discussed previously. Several commonly used medications can alter thyroid hormone secretion. 

T4 and T3 in plasma are reversibly bound to protein, primarily thyroxine-binding globulin (TBG). Only about 0.04% of total T4 and 0.4% of T3 exist in the free form. Many physiologic and pathologic states and drugs affect T4, T3, and thyroid transport. However, the actual levels of free hormone generally remain normal, reflecting feedback control. 

A number of medications have been shown to alter thyroid function and thyroid function tests.Few drugs are associated with the development of clinically significant thyroid disease (amiodarone may be an exception), but difficulty in the interpretation of thyroid function tests results when patients are placed on medications that affect thyroid function testing. In general, drugs do not interfere chemically with the assays for thyroid hormones or TSH. The medications most likely to affect TSH concentrations are glucocorticoids and dopamine (reduced TSH concentrations) and amiodarone (increased TSH concentrations). The most commonly encountered variations in thyroid hormone measurements induced by medications are reduced peripheral conversion of T, to T3 or altered binding of T4 and T3 to carrier proteins. Some of the medications that affect thyroid function tests are shown in Table 52-2. 

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