Dissociation of Thyrotropin and Prolactin Responsiveness to Thyrotropin Releasing Hormone Stimulation in L-Dopa Treated Parkinsonian Patients

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Abstract
The hPRL [human prolactin], hTSH and T3 [triiodothyronine] response to TRH stimulation (200 .mu.g i.v.) were studied in 8 parkinsonian patients under chronic L-dopa-carbidopa therapy. In 6 of the 8 patients studied, treatment was stopped for a period of 2 wk and the TRH stimulation test was repeated under similar experimental conditions. In the L-dopa-carbidopa treated patients basal hTSH levels and the hTSH response to TRH were significantly suppressed. In the 6 patients 2 wk after cessation of treatment, although basal hTSH levels were still suppressed, a normal hTSH response to TRH was observed. Neither the basal T3 and thyroxine concentrations, nor the T3 response to TRH were affected by the L-dopa-carbidopa treatment. Basal hPRL levels as well as the hPRL response to TRH were within the normal range in the 2 groups of patients studied. This study provides further support for a dopaminergic inhibitory action on the hypothalamo-hypophyseal-thyroidal axis (HHTA). The inhibition of basal hTSH secretion and the hTSH response to TRH by L-dopa, suggest that the blocking action of dopamine is exerted at the hypothalamic as well as the pituitary level. Chronic administration of L-dopa did not affect either tonic hPRL secretion or the hPRL response to TRH. The dissociation or response to TRH under the same inhibitory action of dopaminergic stimulation can be interpreted as demonstrating a greater sensitivity of the pituitary thyrotrophs, than the PRL secreting cells, to the blocking effect of dopamine.

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