Effect of L-Dopa on Plasma TSH Levels in Primary Hypothyroidism

Abstract

Plasma TSH responses after an i.v. injection of 100 mg of L-dopa were evaluated by radioimmunoassay in 4 normal euthyroid subjects and in 8 patients with primary hypothyroidism. In agreement with previous results, no variations in plasma TSH levels were observed in the euthyroid subjects. In contrast, in primary hypothyroidism L-dopa induced a biphasic response in plasma TSH. In fact, we observed a transitory increase with a maximum at 30 min (mean ± SEM = 54 ± 18%) followed by a decrease reaching a minimum level of plasma TSH basal values at 90 min (mean ± SEM = 15 ± 6%). These findings demonstrate that the plasma TSH response to L-dopa in primary hypothyroidism is time-dependent. Some speculation on the possible mechanism of this action is presented.

The effect of L-DOPA administration on thyrotropin (TSH) and thyrotropin releasing hormone (TRH) levels in serum in primary or pituitary hypothyroidism.

“…These data confirmed the previous works (Rapoport et al, 1973;Refetoff et al, 1974;Tsukui et al, 1974;Minozzi et al, 1975). T4 and T3 levels in serum did not change after L-DOPA administration in primary or pituitary hypothyroidism.…”

“…The role of catecholamine in thyrotropin releasing hormone (TRH) and thyrotropin (TSH) release has been studied (Brown et al, 1972;Spaulding et al, 1972;Grimm and Reichlin, 1973;Hill-Samli and MacLeod, 1974;Besses et al, 1975;Chen and Meites, 1975;Burrow et al 1977). It has been reported that L-DOPA inhibits TSH release in primary hypothyroidism (Rapoport et al, 1973, Refetoff et al, 1974Tsukui et al, 1974;Minozzi et al, 1975). The mechanism of this inhibitory effect of L-DOPA on TSH release is suggested either directly to pituitary or hypothalamus (Rapoport et al, 1973;Refetoff et al, 1974;Tsukui et al, 1974;Minozzi et al, 1975), but the mechanism is still unclear.…”

“…It has been reported that L-DOPA inhibits TSH release in primary hypothyroidism (Rapoport et al, 1973, Refetoff et al, 1974Tsukui et al, 1974;Minozzi et al, 1975). The mechanism of this inhibitory effect of L-DOPA on TSH release is suggested either directly to pituitary or hypothalamus (Rapoport et al, 1973;Refetoff et al, 1974;Tsukui et al, 1974;Minozzi et al, 1975), but the mechanism is still unclear.…”

Lack of Evidence for an Inhibitory Role Played by Tuberoinfundibular Dopaminergic Neurons on TSH Secretion in the Rat

“…In humans the role of central dopaminergic neu rons requires a more detailed analysis. Tn euthyroid subjects the administration of dop aminergic agonists does not modify the TSH secretion [Collu et al, 1975;Minozzi et al, 1975;Leblanc et al, 1976]. These results ap pear to be in line with those obtained by us in rats.…”

Late Endocrine Effects of L-Dopa, 5-HTP, and 6-OH-Dopa Administered to Neonatal Rats

“…It also blocks the normal in crease in Prl after TRH stimulation without impairing the increase in TSH [Chen and M eites, 1975], Although L-dopa did not alter normal TSH secre tion it was reported to consistently suppress the elevated levels seen in primary hypothyroidism [R efetoff et al, 1974], The serotonin precursor, 5-HTP, has been shown to cause an increase in Prl and TSH [C h en and M eites, 1975], and a decrease in TSH in hypothyroid, but not in euthyroid individuals [M in o z z i et al, 1975]. It has also been shown to cause a decrease in LH and FSH secretion [K am beri, 1973], and an increase in ACTH [Popova et al, 1972] and GH [Lancranjan et al, 1977], A 3rd compound, 6-hydroxy-dopa (6-OH-dopa), a precursor of 6-hydroxydopamine, was selected because when given to the neonatal rat, it crosses the blood brain barrier and produces a 'chemical sympathectomy', and causes depletion of norepinephrine (N E ) and DA, with little effect on 5-HTP, throughout most of the central nervous system (C N S ) [Lytle et al, 1972;C la r k et al, 1972;Z ieher et al, 1973;M c Lean et al, 1976;and A n g e letti, 1971].…”