The parathyroid hormone is not without a counterpart to regulate its effects on the body. Although parathyroid hormone is mainly secreted in response to low calcium and phosphate levels, the body maintains another mechanism to decrease the levels of calcium should it rise beyond normal levels.
What is the Parathyroid Counterpart?
Calcitonin is a peptide hormone composed of 32 amino acids with a molecular weight of about 3400. It is synthesized and secreted by the parafollicular cells or C cells of the thyroid gland. These cells constitute just about 0.1% of the whole thyroid gland.
Calcitonin acts in a way that is the opposite of parathyroid hormone. It tends to decrease plasma calcium concentration before hypercalcemia could occur.
What Stimulates the Release of Calcitonin?
Calcitonin is stimulated by any increase in the plasma calcium concentration. An increase of ten percent of the normal calcium levels could cause a twofold increase in the rate of secretion of calcitonin.
How Does Calcitonin Affect Plasma Calcium Concentration?
It has been observed that calcitonin can decrease plasma calcium concentration quickly. This hormone mainly operates on two mechanisms to exert its effect in bringing plasma calcium concentration down to normal levels.
- Calcitonin has an immediate effect on the activity of osteoclasts and possibly, even on the osteolytic effect of the osteocytic membrane. Calcitonin works immediately to decrease bone resorption by osteoclasts and the pumping of calcium from bone fluid to the extracellular fluid by the osteocytic membrane. This shifts the balance in favor of depositing calcium in the bone instead. This effect is best observed and most especially significant in young animals due to the rapid interchange of absorbed and deposited calcium in the process of skeletal growth.
- Calcitonin exerts a more prolonged effect on osteoclasts, decreasing the formation of new osteoclasts. Osteoclasts secondarily lead to osteoblastic activity, further decreasing the number of osteoblasts. This means that for a longer period of time, there is reduced bone resorption by the osteoclasts and bone deposition by the osteoblasts. This mechanism, however, is only capable of maintaining the plasma calcium concentration for a few hours to a few days at the most.
Calcitonin also appears to have minor effects on intestinal and renal handling of calcium. These effects are also the opposite of the effects of parathyroid hormone on the intestines and the kidneys. However, they appear to be of little significance and are rarely considered.
Calcitonin Has a Weak Effect on Plasma Calcium Concentration in Adults
Despite its counteracting the effects of parathyroid hormone, calcitonin has only a weak effect on plasma calcium concentration in adults. This is because any slight dip in the plasma calcium concentrations leads to powerful stimulations of parathyroid hormone secretion within several hours. Such is the stimulation that it almost overrides the effect of calcitonin completely.
Also, there is very little absorption of calcium occurring in the intestines that even with the effect of calcitonin, calcium absorbed by the body through the gut is still very small that it hardly affects plasma calcium concentrations.
The counterpart of the parathyroid hormone has a greater role in children because children undergo rapid bone development in their growing years. Children deposit and absorb as much as 5 grams of calcium a day, around 5 to 10 times the total calcium in the extracellular fluid. Also, in conditions like Paget’s disease, wherein the osteoclastic activity is greatly increased, calcitonin plays a more active role in reducing calcium absorption.