Clinical Investigations And Tests Of Parathyroid Functions: Urinary Tests

These investigations include urinary calcium, phosphate, hydroxyproline, cyclic-AMP and other relevant tests such as the thiazide and cortisone tests.

Urinary calcium: In health, more than 95.0% of filtered calcium is reabsorbed by the tubules. If the intake of calcium is steady, urinary loss of calcium is less than 3-4 mg/Kg of ideal body weight in 24 hours. In hyperparathyroidism, with sustained hypercalcemia, the dialy urinary loss of calcium exceeds 300 mg. This gives a positive Sulkowitch test.

Urinary Phosphate: Parathyroid hormone facilitates the excretion of phosphate in urine. When the dietary intake of phosphate is 1-1.5 g/day, the daily urinary loss is less than 1g. Dietary phosphate is mostly absorbed and it tends to raise serum phosphate level. The urinary excretion, therefore, varies directly with dietary intake. Normal phosphate clearance (determined by simultaneous measurement of urinary and serum phosphate) is 10.8+2.7 ml/min. The tubular reabsorption of phosphates exceeds 75-85% of the filtered load. Tubular reabsorption is reduced in hyperparathyroidism and thereby phosphate clearance is increased at least by 50%.

Urinary hydroxyproline: Normal adults eliminate 15-24 mg of hydroxyproline in 24 hours. This is increased in hyperparathyroidism and osteomalacia. Other conditions in which urinary hydroxyproline is altered include growth spurt and bone resorption.

Urinary cyclic-AMP: Measurement of urinary c-AMP levels gives useful clues regarding parathyroid function. Normal adults pass 10 micromoles of c-AMP in 24 hours (1.8 to 4.5 nanomoles/dl of glomerular filtrate). In primary hyperparathyroidism, urinary c-AMP levels are high. Administration of PTH raises urinary c-AMP levels are elevated in hypercalcemia secondary to boney metastases.

Thiazide test: This test helps to establish the diagnosis in patients with intermittent hypercalcemia, sometimes seen in primary or ectopic hyperparathyroidism. In normal subjects, administration of 100mg hydrochlorothiazide daily for ten days does not elevate serum calcium level, but in subjects with hyperparathyroidism it is persistently elevated above 11 mg/dl or more.

Stimulation tests of parathyroid function: These tests are employed to distinguish hypoparathyroidism from pseudohypoparathyroidism.

Ellsworth-Howard test: Normal subjects respond to administration of bovine PTH (200- 400 USP units or 50- 80 BPC units) by increasing the urinary phosphate and c-AMP by about 100%. Subjects with hypoparathyroidism show exaggerated increase in these parameters. In patients with pseudohypoparathyroidism, this response is blunted.

Other indirect tests of parathyroid function: Radiographic assessement of the skeleton and bone scans using 99^m technetium help to detect abnormal areas.

Cortisone test: Oral administration of prednisolone in a dose of 30 mg/day for ten days suppresses serum calcium levels in hypercalcemia of malignancy, ectopic hyperparathyroidism, vitamin D intoxication, tuberculosis and sarcoidosis. In primary hyperparathyroidism, serum calcium level does not fall.