The release of hormones from the anterior lobe of the pituitary gland is carefully regulated by three tiers of control mechanisms that include the following:  Tier I: hypothalamic secretion of hypothalamic-regulating hormones. The pituitary gland is under signif-icant control by the hypothalamus, which regulates release of hypothalamic-regulating hormones into the hypophyseal portal veins. The hypothalamic-regulating hormones are produced by the cells of the hypothalamus in response to circulating levels of systemic hormones and impulses from the CNS. These hormones act directly on the highly spe-cific G protein–linked receptors on the plasma mem-branes of cells residing in the anterior lobe of the pituitary gland. Activation of receptors elicits positive or negative signals that affect gene transcription and lead to stimula-tion or inhibition of pituitary hormone secretion. Most of the tropic hormones produced by the anterior lobe of the pitu-itary gland are regulated by polypeptide-releasing hor-mones, with the notable exception of dopamine. Prolactin (PRL) production is primarily regulated by the inhibitory ef-fect of dopamine (i.e., PRL secretion is tonically inhibited by the release of dopamine from the hypothalamus).  Tier II: Paracrine and autocrine secretions of the pituitary cells. Release of hormones from the pituitary gland is also regulated by soluble growth factors and cy-tokines produced by the cells residing in the pituitary gland.  Tier III: Feedback effect of circulating hormones. The level of hormones in the systemic circulation regu-lates the secretion of cells in the anterior lobe of the pituitary gland. This is primary achieved by negative feedback regulation of hormones secreted by the pitu-itary gland by target hormones. For instance, secretion of TSH is inhibited by thyroid hormones produced in the thyroid gland under TSH influence. To better understand the mechanism of negative regula-tion, consider a simple negative feedback system that controls the synthesis and discharge of T3 and T4 thyroid hormones (see Fig. 21.6). The secretion of thyroid hor-mones is controlled by the release of TSH from the anterior lobe of the pituitary gland into the bloodstream. If blood lev-els of T3 and T4 are high, TRH is not produced or released. If blood levels of T3 and T4 are low, the hypothalamus dis-charges TRH into the hypothalamohypophyseal portal sys-tem. Release of TRH stimulates specific cells within the anterior lobe of the pituitary gland to produce TSH, which in turn stimulates the thyroid to produce and release more thyroid hormones. As the thyroid hormone levels rise, the negative feedback system stops the hypothalamus from discharging TRH. Using the same mechanism of negative feedback regulation, thyroid hormones also act on the thy-rotropes in the anterior lobe of the pituitary gland to inhibit their secretion of TSH.
The pituitary gland is composed of glandular epithelial tissue and neural (secretory) tissue.
The pituitary gland [Lat. pituta, phlegm—reflecting its nasopharyngeal origin] is a pea-sized, compound endocrine gland that weighs 0.5 g in males and 1.5 g in multiparous women (i.e., a woman who has given birth two or more times). It is centrally located at the base of the brain, where it lies in a saddle-shaped depression of the sphenoid bone called the sella turcica. A short stalk, the infundibulum, and a vascular network connect the pituitary gland to the hypothalamus.