Parathyroid hormone-related protein (PTHrP) is not strictly a calcium-regulating hormone. It was identified in 1982 as an important PTH-like factor that plays a central role in the pathogenesis of humoral hypercalcemia of malignancy (HHM). PTHrP functions as: (1) a fetal hormone, (2)a paracrine factor in many fetal and adult tissues, and (3) an abnormal hormone in adults with HHM. Therefore, PTHrP is a polyhormone with multiple biologically active regions.

The two-site IRMA and N-terminal RIA used to measure human PTHrP are useful for measurement of biologically active PTHrP in the dog and cat, because of the high degree of sequence homology between species. The assay is very specific for PTHrP, with no crossreactivity noted to PTH, or any PTH fragment.

PTHrP is very susceptible to degradation by serum proteases, and must be measured in fresh or frozen EDTA plasma. Serum is not recommended for measurement of PTHrP because of the proteolysis that occurs during clotting and sample handling. In a recent study, paired plasma and serum samples were analyzed for PTHrP in approximately 150 dogs with parathyroid-independent hypercalcemia. When plasma was used to measure PTHrP, 35 dogs exhibited a positive PTHrP result, indicating the presence of malignancy. However, when serum was used for PTHrP measurement, only 17 of the previous 35 dogs had a positive PTHrP result.

Metabolites of vitamin D are chemically identical in all species, thus radioimmunoassays developed for use in humans are satisfactory for the measurement in animals. Calcidiol (25-hydroxyvitamin D) concentration is a good indicator of vitamin D ingestion, and can be used to diagnose hypo- or hyper-vitaminosis D.

Either serum or plasma (EDTA or heparin) can be used for measurement of calcidiol. Hemolyzed samples should be avoided, and fasting is recommended but not required. Calcidiol is stable for up to 9 weeks when stored at -20°C, and samples should be shipped on ice. Anticonvulsant use may decrease calcidiol concentration.

Calcitriol (1,25-dihydroxyvitamin D) concentration can be used to detect genetic errors in vitamin D metabolism. Low concentrations are seen in patients with renal failure, and high concentrations are seen in some patients with HHM. The diagnostic utility of calcitriol assay has not been assessed in animals.

Either serum or EDTA plasma can be used for measuring calcitriol. Calcitriol is stable for up to 6 months at -15°C, and samples should be shipped on ice. Bilirubin, cholesterol, triglyceride, hemoglobin, and urea have not been shown to interfere with its measurement.