Wallach's Interpretation of Diagnostic Tests: Pathways to Arriving at a Clinical Diagnosis (1099 page)

Increased In

   Causes of osmotic diarrhea include:

   Bile salt deficiency

   Pancreatic insufficiency

   Celiac/tropical sprue

   Whipple disease

   Intestinal lymphoma

   Medications

   Lactose intolerance

Decreased In

   NA

   Limitations

   Formed stool is not a suitable specimen.

PARATHYROID HORMONE (PTH)

   Definition

   Peptide hormone secreted by parathyroid gland chief cells that controls ionized calcium levels in blood and body fluids by increasing 1,25-dihydroxyvitamin D
₃
(by the kidney), mobilizing calcium from bone (due to increased osteoclast activity), increasing renal tubular resorption of calcium, and reducing renal clearance of calcium, increasing intestinal calcium absorption. The half-life of PTH is <5 minutes. Ionized calcium in blood inhibits PTH secretion. Biologic activity resides in the first 34 terminal amino acids. The intact hormone has 84 amino acids but can be quickly cleaved by proteolysis into smaller less-active fragments. Assay for the intact PTH has largely superseded tests for various PTH fragments. It is important that the PTH assay not cross-react with PTH (7-84) lacking the 6-N-terminal, which has been shown to be a weak antagonist to PTH activity and may lower serum and plasma calcium levels.