(a) Exocrine gland. A gland which possesses a duct and pours its secretion through that duct onto an epithelial surface or into a cavity, rather than into the blood. Examples include the salivary glands and the exocrine part of the pancreas, which supplies digestive secretions. (Note that the pancreas is a composite gland, acting as both exocrine and endocrine gland.)

(b) Endocrine gland. A gland which lacks ducts and is hence called a ductless gland; its secretions, called hormones, pass directly into the blood. The organised endocrine bodies of the human body are the pituitary, pineal, thyroid, adrenal, pancreas, parathyroid, thymus and gonads.

(c) Hormone. Hormones are non-nutrient chemicals which act as intercellular messengers and are produced in trace amounts. This is the current scientific definition; the classical definition described a hormone as a chemical produced by endocrine glands and released into the blood and transported to a distantly located target organ, but the newer definition covers a number of additional molecules such as those secreted by the heart, kidney and gastro-intestinal tract.

Label, from above downwards: hypothalamus and pituitary (base of the brain), pineal (dorsal side of forebrain), thyroid and parathyroid (in the neck, either side of the trachea, the parathyroids on the dorsal surface of the thyroid), thymus (between the lungs behind the sternum), adrenal glands (one above each kidney), pancreas (in the abdomen), and the gonads — testis in the scrotal sac in males, ovaries in the abdomen in females. In addition, the gastrointestinal tract, liver, kidney and heart also produce hormones.

Note that (e) is the odd one out: its target is not a gland at all but a pigment-containing cell of the skin.

(a) Parathyroid hormone (PTH). A peptide hormone from the four parathyroid glands, whose secretion is regulated by the circulating levels of calcium ions. PTH increases the Ca²⁺ levels in the blood by three routes: it acts on bones and stimulates bone resorption (dissolution/demineralisation); it stimulates reabsorption of Ca²⁺ by the renal tubules; and it increases Ca²⁺ absorption from the digested food. PTH is thus a hypercalcemic hormone, and along with thyrocalcitonin it plays a significant role in calcium balance in the body.

(b) Thyroid hormones. The follicular cells secrete thyroxine (T₄) and triiodothyronine (T₃), whose synthesis requires dietary iodine. They play an important role in the regulation of the basal metabolic rate; support the process of red blood cell formation; control the metabolism of carbohydrates, proteins and fats; and influence the maintenance of water and electrolyte balance. The chapter summary adds their role in the development and maturation of the central neural system and in the menstrual cycle. The thyroid also secretes the protein hormone thyrocalcitonin, which regulates blood calcium levels by decreasing them.

(c) Thymosins. Peptide hormones of the thymus gland. They play a major role in the differentiation of T-lymphocytes, which provide cell-mediated immunity, and they also promote the production of antibodies to provide humoral immunity. The thymus is degenerated in old individuals, so thymosin production decreases and the immune responses of old persons become weak.

(d) Androgens. Produced by the Leydig or interstitial cells of the testis, mainly as testosterone. They regulate the development, maturation and functions of the male accessory sex organs (epididymis, vas deferens, seminal vesicles, prostate gland, urethra); stimulate muscular growth, growth of facial and axillary hair, aggressiveness and low pitch of voice; play a major stimulatory role in spermatogenesis; act on the central neural system to influence male sexual behaviour (libido); and produce anabolic effects on protein and carbohydrate metabolism. The summary adds erythropoiesis.

(e) Estrogens. Steroid hormones synthesised and secreted mainly by the growing ovarian follicles. They stimulate the growth and activities of female secondary sex organs, the development of growing ovarian follicles, the appearance of female secondary sex characters (such as a high pitch of voice) and mammary gland development; they also regulate female sexual behaviour.

(f) Insulin and Glucagon. Both are peptide hormones of the Islets of Langerhans, and together they maintain glucose homeostasis in the blood.

Glucagon (from α-cells) acts mainly on the liver cells and stimulates glycogenolysis, raising blood sugar; it also stimulates gluconeogenesis and reduces cellular glucose uptake and utilisation. It is therefore a hyperglycemic hormone.

Insulin (from β-cells) acts mainly on hepatocytes and adipocytes, enhancing cellular glucose uptake and utilisation so that glucose moves rapidly from blood into these cells, and stimulating glycogenesis. It is therefore hypoglycemic. Prolonged hyperglycemia leads to diabetes mellitus.

(a) Diabetes mellitus — deficiency of insulin (and/or insulin resistance). Insulin normally enhances cellular glucose uptake and utilisation and stimulates glycogenesis, lowering blood glucose. Its deficiency leads to prolonged hyperglycemia, which is associated with loss of glucose through urine and the formation of harmful compounds known as ketone bodies. Diabetic patients are successfully treated with insulin therapy.

(b) Goitre — deficiency of thyroid hormones (thyroxine, T₄, and triiodothyronine, T₃), arising from deficiency of iodine in the diet. Iodine is essential for the normal rate of hormone synthesis in the thyroid, so dietary iodine deficiency results in hypothyroidism and enlargement of the thyroid gland, commonly called goitre.

(c) Cretinism — deficiency of thyroid hormones during pregnancy / in the growing child. Hypothyroidism during pregnancy causes defective development and maturation of the growing baby, leading to stunted growth (cretinism), mental retardation, low intelligence quotient, abnormal skin, deaf-mutism and similar effects.

Note that (b) and (c) share the same hormonal deficiency; what differs is when it occurs. In an adult it enlarges the gland and lowers the metabolic rate; during development it produces irreversible defects of growth and of the central neural system.

Origin and control. FSH (follicle stimulating hormone) is one of the six hormones produced by the pars distalis of the pituitary. Its synthesis and release are stimulated by the hypothalamic releasing hormone GnRH, which reaches the anterior pituitary through the portal circulatory system.

Chemical nature and receptor. FSH is a protein (glycoprotein) hormone, so it belongs to the group of peptide, polypeptide and protein hormones. Such hormones bind to membrane-bound receptors on the cell membrane of the target cells, and they normally do not enter the target cell.

Steps of action.

1. FSH is carried in the blood and reaches all tissues, but binds only where its specific receptor exists, since hormone receptors are located in the target tissues only and each receptor is specific to one hormone.
2. Binding forms a hormone-receptor complex at the cell membrane of the target cell in the gonad.
3. The complex generates second messengers (e.g. cyclic AMP, IP₃, Ca⁺⁺) inside the cell.
4. The second messengers regulate cellular metabolism, producing biochemical changes in the target tissue.
5. These cumulative biochemical actions result in physiological and developmental effects.

The resulting effects. In females, FSH stimulates the growth and development of the ovarian follicles. In males, FSH together with androgens regulates spermatogenesis.

Answer: (a)–(ii), (b)–(iv), (c)–(i), (d)–(iii).

Note that (c) and (d) are two consecutive steps of one chain: GnRH from the hypothalamus stimulates the pituitary synthesis and release of the gonadotrophins, of which LH is one.