Effect of Thyroid Hormone Excess and Deficiency on Serum Thyrotropin in Rats Immunized Passively with Antiserum to Somatostatin (39537)

Passive immunization of rats with antiserum to somatostatin (anti SS) caused about a 2 fold increase in the basal serum TSH levels and a 2.5 fold rise in serum TSH after TRH, as compared to rats treated with normal sheep serum (NSS). When the basal and the TRH stimulated TSH levels were suppressed to about half the control levels with 100 ng of T₃/100 g BW, the neutralization of the endogenous somatostatin secretion with anti SS significantly increased the basal as well as the post TRH TSH levels. After administration of 200 ng of T₃/100 g BW, the basal TSH levels in the anti SS and NSS treated animals were the same, but, in response to TRH, the TSH levels rose significantly higher in the anti SS treated group. When higher doses of T₃ (1 and 10 μg/100 g BW) were given, anti SS failed to effect the basal or the post TRH TSH levels. The elevated TSH levels 1 and 3 days after thyroidectomy were further increased by treatment with anti SS as compared to NSS. The highly elevated level of TSH on the seventh day after thyroidectomy was, however, not affected by anti SS. These findings indicate that somatostatin modulates the TSH secretion rate under physiological conditions and can be considered with TRH and thyroid hormones as another regulator of TSH secretion.