Impaired prolactin response to arginine in patients with hyperthyroidism.
OBJECTIVE
Reduced PRL responses to TRH or dopamine antagonists have been described in hyperthyroid patients. Arginine stimulates PRL secretion through pathways other than the activation of TRH receptors or dopamine-dependent mechanisms. We therefore investigated PRL responses to arginine in patients with hyperthyroidism.


METHODS
L-Arginine (30 g infused over 30 minutes) was administered at time zero.


METHODS
Sixteen patients with untreated hyperthyroidism due to Graves' disease (8 female and 8 male), with a mean age (+/- SE) of 31.3 +/- 1.4 years (range 23-42), and 12 normal subjects (6 female and 6 male, ages 30.1 +/- 2.1 years, range 22-47) were studied.


METHODS
Prolactin was measured by RIA between -30 and 120 minutes, at 15-minute intervals.


RESULTS
Basal PRL levels were similar in the hyperthyroid patients and normal control subjects. The hyperthyroid women showed blunted PRL responses compared to normal women (peak PRL levels, 364 +/- 44 mU/l, vs 760 +/- 156, P < 0.02). PRL responses to arginine, small but clearly detectable in normal men, were completely abolished in hyperthyroid men (peak PRL levels, 248 +/- 48 mU/l, vs 112 +/- 14, P < 0.01).


CONCLUSIONS
PRL responses to arginine are impaired in hyperthyroid patients. Therefore, arginine should be added to the list of PRL stimuli whose responses are blunted in hyperthyroidism. Inhibition of PRL gene expression, and thus reduced pituitary PRL synthesis and storage, may explain why PRL responses to all secretagogues are reduced in these patients.
Dipartimento di Fisiopatologia Clinica, Università di Torino, Italy.
E
E
Ciccarelli
M
M
Zini
S
S
Grottoli
P
P
Razzore
I
I
Portioli
R
R
Valcavi
D000328:Adult; D001120:Arginine / Q000008:administration & dosage*; D005260:Female; D006801:Humans; D006980:Hyperthyroidism / Q000097:blood*; D008297:Male; D011388:Prolactin / Q000097:blood*; D011863:Radioimmunoassay
D011388:Prolactin; D001120:Arginine