Concepts: Pituitary, pineal, thyroid and parathyroid

2.1 The pituitary gland

The pituitary gland sits in a bony cavity of the skull called the sella turcica and is attached to the hypothalamus by a stalk. It is divided into two parts. The adenohypophysis has two portions, the pars distalis and the pars intermedia, and the pars distalis is commonly called the anterior pituitary. The neurohypophysis, also called the pars nervosa, is the posterior pituitary.

The anterior pituitary produces growth hormone, prolactin, thyroid stimulating hormone, adrenocorticotrophic hormone, luteinizing hormone and follicle stimulating hormone. Growth hormone promotes the growth of the body by stimulating protein synthesis and the growth of bones and muscles; too much of it in childhood produces gigantism, too much in an adult produces acromegaly with a heavy face and enlarged hands and feet, and too little produces pituitary dwarfism. Prolactin acts on the mammary glands and promotes the formation of milk. Thyroid stimulating hormone makes the thyroid gland produce its hormones. Adrenocorticotrophic hormone makes the adrenal cortex release its glucocorticoids. Luteinizing hormone and follicle stimulating hormone are together called gonadotrophins because they act on the gonads. In the male, luteinizing hormone makes the Leydig cells of the testis produce androgens, and follicle stimulating hormone supports spermatogenesis. In the female, luteinizing hormone triggers ovulation and maintains the corpus luteum, while follicle stimulating hormone stimulates the growth of the ovarian follicles.

The pars intermedia secretes only one hormone, melanocyte stimulating hormone, which acts on the melanocytes and controls the colour of the skin. In humans this part is almost merged with the pars distalis. The posterior pituitary stores and releases two hormones, oxytocin and vasopressin, but both are made in the hypothalamus and only stored in the posterior pituitary. Oxytocin acts on the smooth muscles of the body and is important in the contraction of the uterus at the time of childbirth and in the ejection of milk from the mammary glands. Vasopressin, also called antidiuretic hormone, acts mainly on the kidney and increases the reabsorption of water and electrolytes from the distal tubules, so it reduces the loss of water through urine. A deficiency of vasopressin makes the kidney unable to conserve water, and the resulting condition is diabetes insipidus.

2.2 The pineal gland

This small gland lies on the dorsal side of the forebrain. Its hormone is melatonin, and melatonin sets the twenty four hour rhythm of the body. It helps to maintain the normal rhythm of the sleep and wake cycle and of body temperature, and it also influences metabolism, pigmentation, the menstrual cycle and the defence capability of the body.

2.3 The thyroid gland

The thyroid gland has two lobes, one on either side of the trachea, connected by a thin flap of connective tissue called the isthmus. Its tissue is made of follicles and stromal tissue. Each follicle is lined by follicular cells and encloses a cavity. Within the follicle, the follicular cells produce thyroxine (T4) and triiodothyronine (T3), and the gland needs iodine to build them. These two hormones set the pace at which the body burns carbohydrates, proteins and fats, and so they fix the basal metabolic rate. They also support the growth and development of the body, and they influence the activity of the nervous system. The thyroid gland also produces a protein hormone called thyrocalcitonin, or TCT, which lowers the level of calcium in the blood.

Deficiency of iodine in the diet reduces the synthesis of thyroid hormones, and this hypothyroidism causes the thyroid gland to enlarge, a condition called goitre. Hypothyroidism during pregnancy damages the development and maturation of the growing baby, which leads to stunted growth and mental retardation, and this condition is called cretinism. Excess production of thyroid hormones is hyperthyroidism, and exophthalmic goitre is a form of it in which the eyes bulge out of their sockets along with the other signs of a high metabolic rate.

2.4 The parathyroid glands

The parathyroid glands are four small glands present on the posterior part of the thyroid gland. They secrete parathyroid hormone, also called parathormone. Parathyroid hormone raises the level of calcium in the blood by stimulating the breakdown of bone tissue, by increasing the reabsorption of calcium in the renal tubules and by increasing the absorption of calcium from the digested food. Its action is opposite to that of thyrocalcitonin, and the two hormones together keep the blood calcium within a narrow range, because calcium is needed for clotting, for muscle contraction and for the formation of bone.

2.4.1 Practice from the NCERT exercises, questions 3, 4 and 5

Exercise 3 (part). The hypothalamus secretes releasing and inhibiting hormones; the pituitary secretes growth hormone, prolactin, thyroid stimulating hormone, adrenocorticotrophic hormone, luteinizing hormone, follicle stimulating hormone, melanocyte stimulating hormone, oxytocin and vasopressin; the thyroid secretes thyroxine, triiodothyronine and thyrocalcitonin; and the parathyroid secretes parathyroid hormone.

Exercise 4. Hypothalamic hormones act on the pituitary. Thyrotrophin acts on the thyroid. Corticotrophin acts on the adrenal cortex. The gonadotrophins act on the gonads. Melanotrophin acts on the melanocytes of the skin.

Exercise 5. Parathyroid hormone raises blood calcium by acting on bone, kidney and the intestine. Thyroid hormones control the basal metabolic rate and the growth of the body. Thymosins develop the cell mediated immunity of the body.