Which drug can lead to clinical signs of hypothyroidism?

Trimethoprim-sulfamethoxazole (TMS) has been associated with inducing clinical signs of hypothyroidism in certain circumstances. This occurs because one component of TMS, sulfamethoxazole, can inhibit thyroid hormone synthesis due to its structural similarity to para-aminobenzoic acid (PABA), which is involved in the production of thyroid hormones. In some cases, this inhibition can lead to decreased levels of thyroid hormones in the body, manifesting as clinical signs of hypothyroidism.

In contrast, the other drugs listed, such as ciprofloxacin, enrofloxacin, and amoxicillin, do not have this effect on thyroid hormone synthesis. Although they can cause various side effects, their mechanisms do not interact with thyroid hormone production or metabolism in the way that TMS does. Therefore, TMS is specifically linked to impacts on thyroid function, particularly in animals predisposed to or with underlying thyroid issues.