L-THYROXINE INDUCED TSH SUPPRESSION

Abstract

Thyroid hormones (TH) suppress TSH release not only as a direct effect on the pituitary, but also by inhibiting hypothalamic TRH. Endogeneous somatostatin, dopamin and calcium seem also to play a role. 25 adolescents (5 M, 20 F) receiving high doses of 1-thyroxine (3 μg/Kg/day) for endemic goiter were subdivided in 3 groups (A,B,C) and studied in two phases: in the first phase a TRH test (200 μg i.v.) was performed and TSH, PRL and GH samples taken at time 0′, 20′, 40′ and 60′. In second phase (a week later) a similar TRH test was repeated, but group A (9 pts) was given 60 mg pyridostigmine bromide (a cholinergic agonist) p.os 60′ before, group B (7 pts) received 10 mg metoclopramide (a dopamine antagonist) p.os also 60′ before, and group C (9 pts) 40 mg of verapamil (a calcium antagonist) thrice daily for one week. Basal TH were: T3 1.8±0.7 ng/ml (nv 0.8-2), T4 12.3±3.1 μ/dl (nv 4.5-12). fT3 4.8±2.3 pg/ml (nv 2.5-6), fT4 2.5±1.0 ng/dl (nv 0.8-1.9). Results (basal and peak of 1st TRH Versus 2nd TRH): in group A TSH was 0.3±0.3/0.5±0.5 μU/ml vs 0.2±0.2/0.6±0.4 μU/ml (NS). PRL was 8.9±8/22.4±11.6 ng/ml vs 6.8±6.1/23.3±12.4 ng/ml (NS), while a significant rise of GH 0.5±0.6/2.5±3.3 ng/ml vs 3.3±4.7/11.7±9.2 ng/ml (p< 0.025 for the difference between the peaks) was obtained during the 2nd phase and thus confirming the inhibition of somatostatinergic tone. In group B TSH was <0.1/0.2±0.3 vs <0.1/0.2±0.2 μU/ml (NS), a significantly higher rise of PRL was observed during the 2nd phase 10.4±3.3/21.6±12.6 ng/ml vs 9.4±3.9/58.4±17.1 ng/ml (p<0.001 for the difference between the peaks) due to a lowered dopaminergic tone, while no response of GH was seen 2.9±4.5/4.6±6.4 ng/ml vs 2.5±3.3/2.4±2.2 ng/ml (NS). In group C TSH was 0.1/0.2±0.3 vs 0.1/0.3±0.5 μU/ml, PRL 9.6±6/39.4±22.6 ng/ml vs 11.1±4.6/50±33.3 ng/ml and GH 3.5±1.9/4.1±2.4 ng/ml vs 4.4±3.1/4.1±2.3 ng/ml (NS for all).Conclusion: our results would confirm the hypothesis that the direct effect of the thyroid hormones on the TSH secretion seems to be more important than that mediated by somatostatin, dopamin and/or calcium.