PTH is a hormone secreted by the parathyroids, 3 to 5 small glands located in the neck on the surface of the thyroid. The role of PTH is to ensure there is sufficient calcium in the blood for bone growth, muscular contraction, etc. PTH is secreted when calcium levels drop, and its secretion stops when calcium levels are above the body’s needs.
Levels of PTH are interpreted mainly in light of calcium and phosphorus levels and proper kidney function (creatinine/eGFR). PTH combined with high calcium levels indicate a usually benign overactivity of the parathyroid glands (primary hyperparathyroidism).
Low or low-normal PTH levels combined with high calcium levels indicate that the high calcium levels are due to other causes: cancer, vitamin D toxicity, hyperthyroidism, etc. High PTH levels combined with low or low-normal calcium levels (secondary hypothyroidism) indicate that the parathyroids are properly responding to a calcium deficiency (kidney failure, high phosphorus levels, vitamin D or calcium deficiency, etc.). Low HTP levels combined with low calcium levels indicate that the parathyroids are unable to produce sufficient PTH (primary hypoparathyroidism) following, for example, an autoimmune disease or their removal during thyroid surgery.