(a) Parathyroid hormone (PTH)
A hypercalcemic hormone — it increases blood \(\displaystyle Ca^{2+}\) levels.
Acts on bone and stimulates bone resorption (dissolution/demineralisation).
Stimulates reabsorption of \(\displaystyle Ca^{2+}\) by the renal tubules and increases \(\displaystyle Ca^{2+}\) absorption from digested food.
Along with thyrocalcitonin (TCT), it maintains calcium balance in the body; its own secretion is regulated by circulating calcium levels.
(b) Thyroid hormones (\(\displaystyle T_{4}\) and \(\displaystyle T_{3}\))
Regulate the basal metabolic rate.
Support red blood cell formation (erythropoiesis).
Control the metabolism of carbohydrates, proteins and fats.
Influence the maintenance of water and electrolyte balance, and the development and maturation of the central neural system; in adult women, deficiency makes the menstrual cycle irregular.
The thyroid also secretes the protein hormone thyrocalcitonin (TCT), which regulates blood calcium by lowering it.
(c) Thymosins
Peptide hormones of the thymus that build up the immune system.
Cause differentiation of T-lymphocytes, which provide cell-mediated immunity.
Promote production of antibodies, giving humoral immunity.
The thymus degenerates in old age, so thymosin output falls and immune responses of old persons become weak.
(d) Androgens (mainly testosterone)
Regulate development, maturation and functions of the male accessory sex organs — epididymis, vas deferens, seminal vesicles, prostate gland, urethra.
Stimulate the male secondary sex characters: muscular growth, facial and axillary hair, aggressiveness, low pitch of voice.
Play a major stimulatory role in spermatogenesis.
Act on the central neural system and influence male sexual behaviour (libido).
Produce anabolic effects on protein and carbohydrate metabolism.
(e) Estrogens
Stimulate growth and activities of the female secondary sex organs.
Cause development of the growing ovarian follicles.
Bring about the female secondary sex characters, e.g. high pitch of voice.
Stimulate mammary gland development.
Regulate female sexual behaviour.
(f) Insulin and Glucagon — jointly maintain
glucose homeostasis
Insulin is a hypoglycemic hormone from the β-cells: it acts mainly on hepatocytes and adipocytes, enhances cellular glucose uptake and utilisation, and stimulates glycogenesis (glucose → glycogen), so blood glucose falls.
Prolonged hyperglycemia from insulin deficiency/resistance leads to diabetes mellitus, with loss of glucose in urine and formation of ketone bodies.
Glucagon is a hyperglycemic hormone from the α-cells: it acts mainly on the liver cells, stimulating glycogenolysis and gluconeogenesis, and reduces cellular glucose uptake and utilisation, so blood glucose rises.
