Thyroxine Replacement Requirements in Hypothyroid Patients Receiving Phenytoin

Excerpt

Patients receiving phenytoin have lower levels of serum thyroxine (T₄), free T₄, triiodothyronine (T₃), free T₃, and reverse T₃ than controls. Values for serum thyrotropin (TSH) are normal, however, and these patients remain clinically euthyroid (1-6). We report a patient with primary hypothyroidism on long-term thyroxine replacement who developed clinical hypothroidism after starting phenytoin.

A 32-year-old woman consulted her physician in February 1982 because of cold intolerance and hypersomnolence. At age 13 she had been started on phenytoin for seizures; the drug was discontinued at age 23 after 10 years without a seizure. At age 28 she developed fatigue 2