The following points highlight the three major abnormalities of parathyroid function. The abnormalities are: 1. Hypoparathyroidism 2. Hyperparathyroidism 3. Deficiency of 1, 25(OH)2-D3.
Parathyroid Function: Abnormality # 1. Hypoparathyroidism:
a. The symptoms of hypoparathyroidism are muscular weakness, tetany and irritability.
b. In case, hypothyroidism begins early in childhood, there may be stunting of growth, defective tooth development, and mental retardation.
c. Serum calcium is low, serum phosphate is increased, urinary calcium is low, serum magnesium and hydroxyproline levels are decreased.
d. Calcium, parathyroid hormone, and vitamin D precursors are used in the treatment of hypoparathyroidism.
Parathyroid Function: Abnormality # 2. Hyperparathyroidism:
a. Hyperparathyroidism occurs due to a tumor of the gland. The symptoms of hyperparathyroidism are hypercalcemia, decalcification of bones causing pain and deformities, renal stones.
b. Serum phosphorus is low, serum alkaline phosphatase activity is increased, and excretion of calcium in urine is highly increased.
c. Removal of-tumor brings about prompt relief and injection of calcitonin helps to reduce serum calcium level.
Parathyroid Function: Abnormality # 3. Deficiency of 1, 25(OH)2-D3:
It stimulates intestinal absorption of calcium and phosphate.
a. 1, 25(OH)2-D3 is the only hormone that can promote the translocation of calcium against the concentration gradient which exists across the intestinal cell membrane. If controls the extracellular fluid Ca2+ in spite of marked fluctuations of the calcium content of food.
b. It maintains a proper concentration of calcium and phosphate for deposition on the collagen fibrils in bone.
c. In 1, 25(OH)2-D3 deficiency, new bone formation slows and bone re-modelling is also impaired.
d. It also enhances the actions of parathyroid hormones (PTH) on renal calcium reabsorption.
1, 25(OH)2-D3 acts at the cellular level just like steroid hormones:
a. There is nuclear accumulation of this hormone in pancreatic islet cells; some brain cells; some cells in the pituitary, ovary, testis, placenta, uterus, mammary gland, and thymus.
b. The 1, 25(OH)2 D, receptor is a member of the steroid receptor family. The receptor has a DNA-binding domain that appears to contain the zinc finger motif characteristic of other steroid receptors.
c. The binding of the 1,25(OH)2-D3 receptor to chromatin in the nucleus suggests that 1, 25 (OH)2-D3 stimulates gene transcription and the formation of specific mRNAs.
d. Type 1 vitamin D-dependent rickets is an inherited autosomal recessive trait characterized by a defect in the conversion of 25(OH)-D3 to 1,25(OH)2-D3.
e. The formation of 1,25(OH)2-D3 is reduced by the loss of substantial renal parenchyma and calcium absorption decreases. Parathyroid hormone is increased due to hypocalcemia.