Roles of Thyroid and adrenal glands in the regulation of gonadotropins secretion during lactation in the rat

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The roles of thyroid and adrenal glands in the regulation of LH and FSH secretion were investigated during lactation in rats. Changes in plasma concentrations of T3 and T4 were measured by RIAs throughout pregnancy and lactation. Plasma concentrations of T3 and T4 gradually decreased in the period of pregnancy and the low levels of both hormones were maintained throughout lactation. Ovariectomy on day 2 of lactation (day of parturition = day 0 of lactation) resulted in no difference in levels of T3 and T4 in the plasma as compared with those of sham-operated animals. Plasma levels of T3 and T4 increased after removal of the litter. These results clearly indicated that lactation is accompanied by hypothyroid state in the rat.In the lactating rat, an increase in plasma levels of corticosterone in response to immobilization stress was suppressed as compared with those of non-lactating rats. Removal of the litter restored the reduced response to immobilization stress. The reduced response of secretion of corticosterone to immobilization stress was accompanied by the hypothyroid state during lactation in the rat. Experimentally induced hypothyroid state in non-lactating female rats also suppressed the response of corticosterone secretion to immobilization stress and the replacement of T4 in the hypothyroid rat restored the response to the stress.These findings suggest that thyroid function well correlated with the function of adrenal cortex and hypothyroidal and hypoadrenal state resulted in hypogonadal state in the lactating rat.