Intact Parathyroid Hormone

The Intact Parathyroid Hormone Test measures the levels of the intact parathyroid hormone in the blood.

The parathyroid hormone is secreted by two pairs of parathyroid glands located behind the thyroid gland. It is secreted when blood calcium levels fall below normal levels (hypocalcemia). Its secretion helps to maintain blood calcium levels in three ways:

·         It stimulates the release of calcium from bones

·         It signals the conversion of Vitamin D from the inactive to the active form in the kidneys since Vitamin D helps in the uptake of calcium from food

·         It also signals the kidneys to reduce excretion of calcium through urine

Parathyroid hormone forms a part of a feedback mechanism which also involves calcium, Vitamin D, phosphate and magnesium. Disruptions in this feedback mechanism affect the normal levels of calcium in the blood and can cause hypercalcemia (high levels of calcium in the blood) or hypocalcemia (low levels of calcium in the blood).

Parathyroid hormone is secreted in a diurnal pattern, with levels reaching their highest during sleep and lowest during midday. Parathyroid hormone secretion also follows a seasonal pattern due to its inverse relationship with Vitamin D. Low levels of Vitamin D are generally observed during winters and in cloudy weather due to low sunlight. This leads to increased secretion of parathyroid hormone to compensate for low Vitamin D. On the other hand, high levels of Vitamin D in summers and sunny weather leads to reduced secretion of parathyroid hormone.

The biologically active complete parathyroid hormone is made up of 84 amino acids and is called intact parathyroid hormone (PTH 1-84). Once released into the blood, it is quickly taken up by the liver and kidneys, and cleaved into fragments of variable sizes called C-terminal fragments. The C-terminal fragments have a longer half-life than the intact hormone and are present in the blood for a longer duration. Levels of the intact hormones are normally lower than the C-terminal fragments but increase in case of calcium deficiency. The Intact Parathyroid Hormone Test measures the levels of the intact parathyroid hormone in the blood.