RESUMO
The effects of phentolamine on the concentrations of TSH and the thyroid hormones in hemithyreoidectomized (hemiTXized) rats have been examined. Phentolamine caused a decrease of TSH level in blood serum 30 min after the injection, suggesting an inhibiting effect of phentolamine on TSH secretion. Thyroxine (T4) concentration decreased after 30 min. While triiodothyronine (T3) fell 60 min after the injection, as either a consequence of the earlier TSH fall or a result of a direct inhibiting effect of phentolamine at the thyroid level. Three repeated injections of phentolamine to hemiTXized rats brought about an increase in TSH level, in comparison with the hemiTXized control. This increase of TSH level may be explained as a "rebound effect" resulting from the earlier strong inhibition of TSH release by phentolamine. Another possible explanation is a longer inhibiting effect of phentolamine at the thyroid level than at the pituitary level. This assumption is supported by T3 concentration which, at the same time, remains still low.
