TSH and prolactin stimulation by the decarboxylase inhibitor benserazide in primary hypothyroidism

200 mg of the 4-aromatic amino acid decarboxylase inhibitor benserazide were administered orally to five patients with primary hypothyroidism. Benserazide induced a significant increase in prolactin and TSH plasma concentration in all subjects. Since the drug blocks the conversion of DOPA to dopamine, these hormonal changes may be due to a reduction in circulating and/or median eminence levels of dopamine.