Hormones Of The Anterior Pituitary And Its Disorders: A Case Study Of The Glycoprotein Hormones
The Anterior Pituitary
Hormones of the anterior pituitary are polypeptides with one or more chains. They can be grouped into three classes:
- Corticotropin group: Corticotropic (Adrenocorticortropic hormone [ACTH], melanocyte stimulating hormone [MSH] and lipotropins [LPH]).
Corticotropin and lipotropins form the major bulk. MSH is present only in small quantities. Endogenous opiod peptides are derived from the lipotropins.
- Glycoprotein hormones: Follicle stimulating hormone (FSH), Luteinising hormone (LS or ICSH) and Thyroid stimulating hormone (TSH).
- Somatotropin mammotropin group: Growth hormone (GH) and prolactin (PRL).
The ACTH is a single chain polypeptide with 39 aminoacids. The hormone can be estimated by radioimmunoassay. Blood levels show considerable varation, values being highest early in the morning (40- 80 pg/ml). The ACTH release is facilitated by the corticotrophin-releasing factors of the hypothalamus and inhibited by increasing levels of circulating plasma cortisol. The ACTH production goes on independent of feedback inhibition and diurnal variation during periods of stress. It exerts action on the adrenal cortex and extra-adrenal tissues. The ACTH promotes the synthesis of glucocorticoids and androgens by the adrenal cortex. Aldosterones secretion is only marginally affected. The ACTH exerts its effect through the adenylate cyclase mechanism.
Other effects of ACTH include lipolysis and increase in pigmentation brought about by stimulation of the melanocytes. Normally, these extra-adrenal effects of ACTH are not pronounced, but when ACTH production is increased as in Addison’s disease and Nelson’s syndrome, pigmentation becomes prominent. The role played by lipotropins in man is not understood.
Anterior Pituitary Hormones
The pituitary glycoprotein hormones are thyroid stimulating hormone (TSH), follicle stimulating hormone (FSH) and luteinizing hormone (LH). The placental glycoprotein hormone chorionic gonadotropin (HCG) resembles LH in function. All these hormones act through the adenylate cyclase mechanism.
Thyroid stimulating hormone (TSH): The TSH acts mainly on the thyroid to increase its vascularity and size. It enhances all aspects of thyroid function such as iodine uptake, synthesis of thyroglobulin and release of thyroxine.
The TSH secretion is increased by TRH and inhibits increasing levels of circulating thyroxine and tri-iodothyronine. The effect of circulating thyroid hormones outweighs the effect of TRH in determining TSH secretion. Plasma TSH levels are about 0 to 5 uU/ml.
Basal TSH levels are considerably increased and TSH response to TRH is amplified in primary hypothyroidism. The TSH levels are very low and often undetectable in primary thyrotixicosis. Moreover, the TSH response to TRH is also negligible.
Gonadotropins: Gonadotropins increase the synthesis and secretion of sex steroids and promote gametogenesis. The pituitary, produces FSH, and LH and chorionic gonadotropin (HCG) is produced by the placenta. FSH and LH secretion occurs in spurts. FSH facilitates the maturation of Graafian follicles by acting on the granulose cells of the ovary. In males, spermatogenesis is stimulated. LH acts on the granulose cells to enhance estrogen production. Acting on the interstitial cells of the testes, it increases the secretion of testosterone. Increase in LH levels in the midcycle phase leads to rupture follicle. HCG also acts similarly to LH. LH is responsible for continued development of the corpus luteum till the placenta takes over this function. LH also maintains the steriodogenic function of the placenta.
GNRH influences the synthesis and secretion of LH and FSH. LH is stimulated to a greater degree than FSH. Secretion of LH and FSH is inhibited by rising levels of estrogens, progesterone and testosterone in blood, except during the pre-ovulatory phase. During the pre-ovulatory phase, a high level of estrogen produces a positive feeback on the hypothalamus and pituitary and this results in excessive production of LH. After menopause and castration, the serum levels of FSH are very high.
© 2014 Funom Theophilus Makama