What drugs affect thyroid hormone concentrations?
Phenobarbital and glucocorticoids cause a “sick euthyroid” pattern of low thyroid hormone levels without an increase in TSH. The mechanism by which phenobarbital exerts its effect are not clear but may include a central component. In the long-term administration of phenobarbital there may even be slight decreases in FT4d and increases in TSH. Glucocorticoids are known to suppress TT4 and FT4, but their effect on FT4d is generally less dramatic suggesting that their principal effect is through alterations in protein binding. Sulfa-containing medications affect the thyroid directly to decrease T4 and T3 production, and cause a consequent increase in TSH. This is a true but reversible hypothyroidism. The pituitary thyroid axis recovers in less than 3 weeks following withdrawal of the medication. Reports of a minimal effect of carprofen (Rimadyl) on TT4 exist. FT4d is less affected. However these studies have not included control animals and the existence of a biologically significant effe
