A case of methimazole-induced hypothyroidism in a patient with endemic goiter: effects of endogenous TSH hyperstimulation after discontinuation of the drug.

Serum thyroid hormone and TSH concentrations were monitored in a patient with multinodular endemic goiter and severe methimazole (MMI) induced hypothyrodism up to 190 days after drug withdrawal. Serum concentrations of TT3, TT4 and TSH returned to normal values at the 6th., the 140th, and the 120th. day respectively. Within the first 20 days after MMI withdrawal the increase of serum T3 levels was correlated with the observed decrease of serum TSH concentrations. Successively T3 values decreased and T4 levels progressively increased. Six months after MMI withdrawal basal serum TSH concentration was normal while an exaggerated response to TRH was observed. We think that this peculiar hormone pattern is due to iodine depletion. In this case TSH hyperstimulation increases predominantly T3 secretion demonstrating the reduced thyroidal ability to produce T4 when hyperstimulated.