Enhanced TSH stimulating effect of TRH by sulpiride in man

Sulpiride, a specific dopaminergic blocker, was administered im to 6 normal male volunteers (100 mg) alone or in association with L-DOPA (500 mg per os, 90 min before sulpiride) or with TRH (400 micrograms iv in bolus, 30 min after sulpiride) and blood samples were obtained for TSH radioimmunoassay at various intervals before and after the treatments. Sulpiride alone produced a slight increase of plasma TSH levels which was inhibited by L-DOPA pre-treatment. The previous administration of sulpiride resulted in a marked increase of the TSH response to TRH. It is suggested that dopamine and TRH may interact to modulate pituitary secretion of both TSH and prolactin, though the hypophyseal thyrotrophs and lactotrophs show a different sensitivity to the common stimulatory and inhibitory substances.