Two elements govern the secretion of the anterior pituitary. One influence, from the hypothalamus is mediated by the hypothalamic hypophysiotrophic hormones, which are elaborated in specialized neurones and reach the pituitary by way of the hypophysial-portal vessels. The other influence is the feedback control exerted by target gland hormones directly on the pituitary, and possibly on the hypothalamus as well. In the case of the pituitary-thyroid axis, thyroid hormone exerts a direct inhibitory effect on TSH secretion, interacting with the stimulatory effects of thryotrophin releasing hormone (TRH). In the rat, TRH excretion in the urine is reduced by thyroid deficiency, suggesting that the thyroid hormones may also be involved in regulating TRH secretion, but we cannot be certain that the effect is on hypothalamic TRH secretion, since TRH also is found in other parts of the brain.
In the case of pituitary-gonad control, estrogens inhibit pituitary responsiveness to luteinizing hormone releasing hormone (LRH) in men, but sensitize the pituitary in women. In men, estrogens stimulate LRH release as inferred from changes in plasma bioassayable luteinizing hormone-releasing factor (LRF) activity, thus suggesting a positive feedback effect of estrogens on the hypothalamus. In women, LRF activity appears in the blood during midcycle, suggesting that midcycle LRH secretion, together with estrogen sensitization of the pituitary, are components of the mechanism underlying the midcycle ovulatory surge.