The parathyroid hormone (PTH) is secreted by the parathyroid gland, four small glands located on the posterior surface of the thyroid gland. The hormone is regulated by the calcium and phosphate levels in the blood and acts primarily on the bone and kidneys to increase circulating levels of calcium. Over- or underactivity of the parathyroid gland ultimately impacts on the blood and urine calcium levels due to an excess or deficiency of parathyroid hormone.
Production of Parathyroid Hormone
The parathyroid hormone (PTH) is synthesized as a preprohormone by the ribosomes of the chief cells of the parathyroid glands. This preprohormone is a long polypeptide chain which is cleaved several times by the endoplasmic reticulum and Golgi apparatus of the cells before being stored in secretory vesicles as the final active form of PTH comprising some 84 amino acids. Smaller peptide fragments of just 34 amino acids are also found in the parathyroid gland and have a similar but longer lasting effect than PTH.
Control of PTH Secretion
The secretion of parathyroid hormone is dependent on the calcium levels of the blood and not the action of any releasing-hormones. This regulation is tightly controlled and by bypassing intermediate regulation by releasing hormones, the parathyroid gland can change the circulating calcium and phosphate calcium levels almost immediately.
Due to its slow and rapid effect on the bones and its action on the kidneys, PTH can change calcium levels in the blood within minutes and sustain it for weeks and months. If there is any chronic deficiency of calcium, the parathyroid gland will expand in size (hypertrophy) to compensate for sustained PTH secretion. Conversely, a persistent elevation in the calcium levels will cause the parathyroid glands to shrink in size.
Functions of Parathyroid Hormone
In order to understand the function of parathyroid hormone, it is important to look at in calcium and phosphate metabolism. About 99% of the body’s calcium and 85% of the body’s phosphate is stored in the bones. These compounds do not only contribute to bone but also play various important functions in the body in much smaller amounts than what is stored in the bones. Even slight changes in the calcium levels will have an immediate effect on the body, while with phosphate levels, changes will not become apparent immediately.
The bone, however, is not the sole factor in regulating calcium and phosphate levels in the blood due to hormonal action. A small quantity of calcium and most phosphate in the food are absorbed from the gut. A significant quantity of both calcium and phosphate are excreted by the kidneys. Without controlling the intake and excretion of calcium and phosphate at the gut and kidneys, the levels of these compounds cannot be adequately regulated.
PTH Effect on the Bone
PTH causes absorption of calcium and phosphate from the bone and its action can be divided into a rapid phase and a slow phase. The rapid phase, which starts within minutes of PTH secretion and lasts for several hours, causes the osteocytes (bone cells) to absorb calcium and phosphate in the bone. The slow phase, which last for days or weeks, increases the number of osteoclasts thereby causing resorption (breakdown) of bone with an increase in circulating calcium and phosphate levels.
PTH Effect on the Kidneys
PTH increases renal tubular reabsorption of calcium from the urine. Without this effect, about 10% of the calcium absorbed from food would be lost in the urine each day. At the same time, PTH increases the excretion of phosphate thereby promoting the loss of phosphate in the urine.
PTH Effect on the Gut
Parathyroid hormone increases absorption of calcium indirectly by converting 25-hydroxycholecalciferol in the kidneys to 1,25-dihydroxycholecalciferol. This is the most active form of vitamin D which is responsible for increasing intestinal absorption of calcium.