11th Class Biology Chemical Coordination and Integration Parathyroid Gland 

(1) Position and structure : These are four in number which are wholly or partially embedded in the dorsal surface of the thyroid gland two glands in each lobe of thyroid gland. Each is oval shaped, small sized \[(5\times 5\text{ }mm)\] and yellow coloured. Histologically, a parathyroid gland is formed of masses of polygonal cell arranged in cords. Endocrine cell are two types principal or chief and oxyphil cells. Parathyroid is endodermal in origin.

(2) Hormones of parathyroid : Active hormone secreted by parathyroids is parathormone (PTH), also called Collip's Hormone (Phillips collip, 1925). It was discovered and purified by Collip in 1925. Its crystals were first prepared by Craig and Ras mussen in 1960. Its molecular structure was worked out by potts and his associates in 1971. The latter is a protein of 84 amino acid monomer. It is a polypeptide hormone. Parathyroids are present in all vertebrates except fishes. Its secretion is stimulated by low level of calcium in blood than normal level through feedback control.

Functions of parathormone : Parathormone is essential for survival, because it significantly contributes to "homeostatis" by regulating the amount of calcium and phosphate ions in ECF. Our body requires an optimum calcium level (10.0 to 11.5 mg per 100mL.) in ECF (total 1000 to 1120 grams in a 70 kg man), because calcium is a key element in many physiological functions like proper permeability of cell membranes, muscular activities, nerve impulse conduction, heartbeat, blood coagulation, bone formation, fertilization of ova, etc. Calcium is most abundant of all minerals found in the body and about 99% of calcium and phosphorous are contained in the bones.

Maintenance of proper calcium level under 'homeostasis' is, in fact, a combined function of parathormone, thyrocalcitonin and vitamin D₃ (cholecalciferol). Parathormone promotes absorption of calcium from food in the intestine and its reabsoption from nephrons in the kidneys. Simultaneously, it accelerates elimination of phosphates in urine (phosphaturic action). Thus, calcium level tends to rise in the ECF due to the effect of parathormone. This calcium is, then, utilized by bone-forming cells – osteoblast – in bone formation under the influence of vitamin D₃. Bones are asymmetrical when first formed. Their unnecessary parts are, therefore, dissolved by bone-eating cells called osteoclasts. This process also proceeds under the influence of parathormone. It results in release of calcium and phosphate in blood.

Vitamin D₃, is a steroid hormone which is first synthesized in an inactive form in skin cells from 7-dehydrocholesterol under the influence of ultraviolet (UV) rays of sunlight. Skin cells release it in blood. Liver cells take it from blood, change in into 25-hydroxycholecalciferol and release back into blood. Finally, the cells of proximal convoluted tubules of nephrons in the kidneys change 25-hydroxycholecalciferol into 1-25-dihydroxycholecalciferol under the influence of parathormone. This last compound is released in blood as active vitamin D₃ named as cholecalciferol (calcitriol).

In addition to its role in bone-remodelling, D₃ also stimulates absorption of Ca²⁺ and Mg²⁺ in intestine. Similarly, parathormone also plays an additional role of stimulating excretion of Na⁺, K⁺ and HCO^–₃, but retarding the excretion of \[M{{g}^{2+}}.\]

Irregularities of parathormones

(1) Hypoparathyroidism (Hyposecretion of  parathormone)

(i) It is rare, However, in undersecretion of parathormone, the level of calcium in ECF falls (hypocalcemia), and that of phosphates rises (hyperphosphatemia). This causes neuromuscular hyperexcitability, excessive perspiration, gooseflesh (raising of hairs and prickly sensation in skin), cooling of hands and feet, painful muscular spasms and convulsions, and trembling.

(ii) Sometimes some skeletal muscles, usually of hands and feet, fail to relax after a contraction, and remain in "sustained contraction". This is called "Tetany". Tetany of larygneal, thoracic, and phrenic muscles, which help in breathing, causes death, because the patient fails to breathe (asphyxia).

(iii) Childhood hypoparthyroidism retards growth, particularly of bones, teeth, hair and brain. Vitamin D is administered to such children.

(2) Hyperparathyroidism (Hypersecretion of parathormone) 

(i) Osteoporosis : Oversecretion of parathormone is rare and occurs usually due to overgrowth of one or more parathyroid glands. It causes demineralization of bones which, therefore, become soft, weak, distorted and fragile. This is called osteoporosis.

(ii) Hypercalcemia : Simultaneously, due to a sharp rise in calcium level in blood and ECF (hypercalcemia) and a sharp fall in phosphate level (hypophosphatemia), muscles and nerves are weakened.

(iii) Hypercalciurea : Calcium is excreted in urine (hypercalciurea), thirst increases owing to copius urination, appetite is lost, constipation and headache become common, and often, kidney stones are formed. The only treatment so far known is removal of extra part of the glands by operation.

Feedback control of secretion of parathormone and thyrocalcitonin : Secretion of these two hormones is continuously regulated by a direct negative feedback. As \[C{{a}^{2+}}\] levels tends to fall, secretion of parathormone increases, but that of thyrocalcitonin decreases. Contrarily, the secretion of parathormone decreases and that of thyrocalcitonin increases when \[C{{a}^{2+}}\] level tends to rise in blood.