The following points highlight the top two types of parathyroid hormones. The types are: 1. Parathormone 2. Calcitonin.
Parathyroid Hormones Type # 1. Parathormone:
Chemistry:
a. It is a polypeptide consisting of 84 amino acids.
b. Its molecular weight is 9,500.
c. This hormone from different species differ only slightly.
d. It is initially synthesized in the chief cells as pre-prohormone.
e. The secreted hormone is degraded rapidly; it has a half-life of about 18 minutes.
The normal level of parathyroid hormone in serum is 150-300 Pg/ml.
Functions:
a. The parathormone stimulates the membrane bound adenylate cyclase causing increased synthesis of cAMP.
b. It mobilizes calcium and phosphorus from bones.
c. It increases serum calcium and lowers the serum phosphorus.
d. It increases urinary excretion of phosphate but decreases excretion of calcium.
e. It elevates serum alkaline phosphatase activity.
f. It increases the absorption of calcium and phosphorus from the intestine.
g. It activates vitamin D in renal tissue by increasing the rate of conversion of 25- hydroxycholecalciferol to 1, 25-di-hydroxycholecalciferol.
h. It increases bone resorption by releasing calcium as well as collagenase, lyosomal enzymes, and hydroxyproline.
i. In the kidney, it affects renal tubular reabsorption of calcium and phosphorus.
j. It may stimulate protein synthesis in the osteoclasts which effect resorption of bone.
Control of Release of Parathormone:
a. The concentrations of parathormone are decreased abruptly by administration of calcium ion and rise when circulating ionized calcium is lowered by the administration of the chelating agent.
b. Calcium loss associated with uremia results in an increase in circulating parathormone.
c. Vitamin A administration decreases parathormone by increasing calcium uptake into the parathyroid gland.
Parathyroid Hormones Type # 2. Calcitonin:
Chemistry:
a. Calcitonin is a polypeptide consisting of 32 amino acids.
b. Its molecular weight is 3,600.
c. There are large variations in the amino acid composition among different species.
d. It is required for biologic activity.
e. The half-life of the secreted calcitonin is about 4-12 minutes.
Functions:
a. It is calcium-lowering hormone originated from the C cells of the thyroid gland.
b. It is directly effective on bone. It inhibits bone resorption and mobilization of calcium and phosphorus from bone.
c. In the kidney, it increases calcium excretion and inhibits synthesis of 1, 25- di-hydroxycholecalciferol without affecting cAMP levels.
d. It decreases the excretion of urinary hydroxyproline by inhibiting the resorption of the organic phase of bone.