Parathyroid Function Tests
Parathyroid hormone is secreted in response to a fall in plasma Ca ++ .
The rise of plasma Ca is achieved through the following:
Direct effect on bone by stimulating bone turnover.
Direct effect on the renal tubules, to enhance Ca ++ reabsorption.
Indirect effect on the small intestine: PTH stimulates the formation of 1, 25dihydroxy
cholecalciferol in the kidney, and this increases Ca ++ absorption from the small intestine.
Hyperparathyroidism:
Primary hyerparathyroidism may be caused by parathyroid hyperplasia adenoma, or
carcinoma. These patients usually present with renal calculi or with metabolic bone disease.
Laboratory findings in this condition are:
Raised serum calcium.
Decreased fasting serum phosphate.
Increased serum alkaline phosphatase activity.
Elevated PTH levels
Secondary hyperparathyroidism develops when long standing hypocalcemia results in
stimulation of parathyroid with development of hyperplasia of the gland. Tertiary
hyperparathyroidism refers to the development of a functioning parathyroid adenoma as a
complication of secondary hyperparathyroidism.
Hypoparathyroidism:
Failure to secrete PTH may be familial or sporadic. It may be a complication of
surgery, or autoimmune process or as a result of infiltration by carcinoma of the thyroid or
other neoplasms.
Laboratory findings in hypoparathyroidism:
Low serum calcium and increased serum phosphate in a patient who does not have renal
disease.
Serum alkaline phosphatase activity is usually normal.
Serum PTH is reduced, and sometimes is undetectable.
Plasma calcium: Calcium is the most abundant mineral in the body. About 99% of the body's
calcium is present in bone where it acts as a reservoir which helps to stabilize Ca ++ in extra
cellular fluid.
In the plasma, calcium is present in three forms, ionized calcium (Ca ++ ), calcium bound
to plasma proteins, and calcium complexed with citrate. The first is the physiologically active
component (50 - 65% of total calcium), and is regulated mainly by parathyroid hormone.
Measurements of total plasma calcium (2.12 – 2.62 mmol/L) is often misleading due to the
effect of changes in plasma albumin, since both are closely associated. It is better to measure
plasma ionized calcium (Ca 2+ ) which is maintained constant by PTH.
Other causes of hyperca1cemia are:
Malignant disease e.g. multiple myeloma.
Excessive calcium absorption e g vit. D overdose.
Drugs e.g. thiazides.
Artifact e.g. excessive venous stasis during venepuncture.
Other causes of hypocalcemia are:
Renal disease (impaired hydroxylation of 25 HCC, acidosis).
Dietary deficiency of calcium or vitamin D.
Hypoproteinemia.
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