Pineal, Thyroid, Parathyroid and Thymus
பீனியல், தைராய்டு, பாராதைராய்டு, தைமஸ்
PTH raises blood calcium; calcitonin (from the thyroid) lowers it. So a statement saying 'PTH decreases Ca2+ in blood' is false — and that is NEET's favourite trap here.
The idea
The pineal gland is located on the dorsal side of the forebrain and secretes melatonin, which plays a very important role in the regulation of the 24-hour (diurnal) rhythm of our body — it helps maintain the normal rhythms of the sleep-wake cycle and body temperature, and also influences metabolism, pigmentation, the menstrual cycle and our defence capability. The thyroid gland is composed of two lobes located on either side of the trachea, interconnected by a thin flap of connective tissue called the isthmus; it is composed of follicles and stromal tissues. Each follicle is composed of follicular cells enclosing a cavity; these cells synthesise two hormones, tetraiodothyronine or thyroxine (T4) and triiodothyronine (T3). Iodine is essential for the normal rate of hormone synthesis in the thyroid; deficiency of iodine in our diet results in hypothyroidism and enlargement of the thyroid gland, commonly called goitre. 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, etc.; in adult women it may cause irregular menstrual cycles. Due to cancer of the thyroid gland or development of nodules, the rate of synthesis of thyroid hormones may increase to abnormally high levels (hyperthyroidism); exophthalmic goitre (Graves' disease) is a form of hyperthyroidism characterised by enlargement of the thyroid gland, protrusion of the eyeballs, increased basal metabolic rate and weight loss. Thyroid hormones regulate the basal metabolic rate, support the formation of red blood cells, control the metabolism of carbohydrates, proteins and fats, and help maintain water and electrolyte balance. The thyroid gland also secretes a protein hormone called thyrocalcitonin (TCT), which regulates blood calcium levels. In humans, four parathyroid glands are present on the back side of the thyroid gland, one pair each in the two lobes. They secrete a peptide hormone called parathyroid hormone (PTH), whose secretion is regulated by circulating levels of calcium ions. PTH increases Ca2+ levels in the blood: it acts on bones and stimulates the process of bone resorption (dissolution/demineralisation), stimulates reabsorption of Ca2+ by the renal tubules and increases Ca2+ absorption from digested food. PTH is thus a hypercalcemic hormone; along with TCT it plays a significant role in calcium balance in the body. The thymus gland is a lobular structure located between the lungs behind the sternum on the ventral side of the aorta. It plays a major role in the development of the immune system and secretes peptide hormones called thymosins, which play a major role in the differentiation of T-lymphocytes (cell-mediated immunity) and promote production of antibodies (humoral immunity). The thymus is degenerated in old individuals, resulting in a decreased production of thymosins, so the immune responses of old persons become weak.
Why it matters
These glands set the body's daily rhythm (pineal), metabolic speed (thyroid), calcium balance (parathyroid and thyroid) and immune training (thymus).
NEET lens
Must-know: melatonin — pineal, diurnal rhythm (sleep-wake, body temperature). Thyroid: T3, T4 (iodine needed); goitre (iodine deficiency); cretinism (hypothyroidism in pregnancy); Graves' disease/exophthalmic goitre (hyperthyroidism); TCT lowers blood Ca2+. Parathyroid: four glands; PTH is hypercalcemic — stimulates bone resorption, renal Ca2+ reabsorption, intestinal Ca2+ absorption. Thymus: thymosins → T-cell differentiation; degenerates with age. PTH and calcitonin are antagonistic.
Clinical significance
India's iodised-salt programme has sharply reduced goitre and cretinism. Thyroid function tests are routine in pregnancy because untreated hypothyroidism can harm the baby's brain development.
Introduction
Four smaller endocrine glands: the pineal (rhythms), thyroid (metabolism, calcium), parathyroid (calcium) and thymus (immunity).
Core concept
Pineal: dorsal side of forebrain; melatonin → 24-hour (diurnal) rhythm, sleep-wake cycle, body temperature; also metabolism, pigmentation, menstrual cycle, defence capability. Thyroid: two lobes on either side of trachea + isthmus; follicles → T4 (thyroxine), T3; needs iodine. Iodine deficiency → hypothyroidism, goitre. Hypothyroidism in pregnancy → cretinism (stunted growth, mental retardation, low IQ, abnormal skin, deaf-mutism); in adult women → irregular menstrual cycles. Hyperthyroidism → exophthalmic goitre/Graves' disease (enlarged thyroid, protruding eyeballs, ↑BMR, weight loss). Thyroid hormones: BMR, RBC formation, carbohydrate/protein/fat metabolism, water and electrolyte balance. TCT (thyrocalcitonin): protein hormone, regulates (lowers) blood Ca2+. Parathyroid: 4 glands on back of thyroid; PTH (peptide), regulated by blood Ca2+; ↑ blood Ca2+ by bone resorption, renal tubular Ca2+ reabsorption and ↑ absorption from digested food → hypercalcemic. Thymus: between lungs behind sternum, ventral to aorta; thymosins → T-lymphocyte differentiation (cell-mediated) and antibody production (humoral); degenerates in old age → weak immunity.
Go deeper
PTH does not affect carbohydrate metabolism. Its three actions (bone, kidney, gut) all raise blood calcium.
From Class 10
Class 10 linked iodine deficiency with goitre. NEET adds calcium control by PTH and calcitonin, and the pineal and thymus.