Effects of Other Pharmacological Agents on Thyroid Function

Pharmacological agents can modulate thyroid function at all levels, as illustrated in Fig. 1. Glucocorticoids, somatostatin, dopaminergic (dopamine, bromocriptine) and antidopaminergic agents (e.g. metoclopramide, droperidol) can all alter pituitary thyroid-stimulating hormone (TSH) secretion when present in pharmacological doses, as discussed in Chap. 2, this volume. Iodine, antithyroid drugs, lithium and amiodarone all have effects on thyroidal hormone production and these agents are discussed in detail in Chaps. 7–10, this volume. Steroids, diuretics, heparin and nonsteroidal anti-inflammatory drugs have effects on thyroid hormone transport, thereby altering the total, but not free, hormone levels, as detailed in Chap. 5, this volume. However, additional substances are rare but important modulators of thyroid function and thyroid hormone metabolism, and they are the subject of the present chapter. At the level of the thyroid, sulphonamides, op’DDD, aminogluthethimide, cyanate, selenium, noradrenaline, cytokines and growth hormone influence hormonogenesis at various steps (Fig. 2). The early steps involved in thyroid hormone metabolism through a system of three different organ-specific deiodinases are subject to interference by iodinated drugs, propylthiouracil (PTU), β-adrenoceptor blockers and glucocorticoids. In addition, the final degradation of thyroid hormones can be altered by a variety of microsomal enzyme inducers, such as antiepileptics and rifampicin. These interactions are of particular interest when dealing with patients on exogenous thyroid hormone therapy. Finally, knowledge about the drugs known to interfere with intestinal thyroxine absorption (sucralfate, ferrous sulphate, cholestyramine, colestipol, aluminum hydroxide) is of equal importance in the management of the latter group of patients.