Parathyroid Hormone (PTH) and Ionized Calcium (iCa)

Parathyroid hormone (PTH) and ionised calcium (iCa) measurements are the mainstay of calcium investigations and their combined measurement should be considered in both hypo- and hypercalcaemic situations where the cause is not immediately obvious. Neither total nor albumin corrected calcium measurements give a true reflection of calcium status in all cases. This is particularly so in animals with compromised renal function.

The combination of iCa and PTH measurement allow us to distinguish between four main categories of calcium regulation disorders:

  1. Primary hyperparathyroidism (parathyroid dependent hypercalcaemia) – functional parathyroid neoplasia.
  2. Parathyroid suppression (parathyroid independent hypercalcaemia) – hypercalcaemia of malignancy, vitamin D toxicosis (cholecalciferol rodenticide, calcitriol, Dovonex), feline idiopathic hypercalcaemia* and granulomatous disease processes.
  3. Primary hypoparathyroidism (parathyroid dependent hypocalcaemia) – parathyroid gland destruction by inflammation or surgery.
  4. Secondary hyperparathyroidism (parathyroid independent hypocalcaemia) – renal failure, calcium losses (e.g. pancreatitis, equine diarrhoea/colic), dietary deficiency, rickets, hyperadrenocorticism, hyperthyroidism.

* As it’s name suggests, there is currently limited understanding of feline idiopathic hypercalcaemia. In some studies there is a suggested association with urinary acidifying diets. In others, calcium oxalate crystalluria has been reported and some amelioration achieved by a change to high fibre diets. Some authors believe the condition to be relatively benign, while others believe that the condition promotes the onset of renal dysfunction. Low-dose-glucocorticoids have been suggested as a therapy if there is concern about progressive disease.

PTH/Ionized Calcium Interpretation Chart

Ionized Calcium Interpretation Chart

The chart illustrates the diagnostic categorization of calcium disorders using PTH and iCa alone. The chart as depicted applies well to dogs but the lower and smaller feline PTH reference range makes interpretation sometimes a little less clear-cut in cats.

Sample preparation is critical for the accurate analysis of PTH, see procedure below.

An Ionised calcium sample should also be taken at the same time as the PTH sample as this gives an accurate indication of the calcium status at that time.

PTH Sample Preparation

Submitted sample must be EDTA plasma or EDTA plasma with Aprotinin

  1. Request a transport pack for delivery to the laboratory.
  2. Take the blood sample into a fridge cooled EDTA or Aprotinin EDTA tube (supplied in pack) which has been cooled in the fridge. Note: To avoid agglutination of samples in the assay, if possible, half fill several EDTA tubes with blood to ensure a high concentration of EDTA, especially if very high levels of calcium have been observed.
  3. Mix very well but gently and centrifuge as quickly as possible. Note: If the Aprotinin EDTA tube will not fit into your (ideally in a refrigerated centrifuge, then carefully transfer the well mixed blood into cooled tubes that do fit into the centrifuge.
  4. Transfer the plasma into a cooled plastic (not glass!) PLAIN (or EDTA tubetube if agglutination has occurred in a previous sample) which has been cooled in the fridge.
  5. Immediately freeze (<-10°C) the plasma sample and keep frozen until dispatch in the transport pack. Freeze the plasma sample first, separately, before placing in the frozen pack for transport.Samples are stable for several weeks frozen.
  6. 6. Mark the tube with “EDTA plasma” and animal/owner name.

Ionised Calcium (iCa)

Submitted sample should be separated serum. EDTA plasma is not acceptable, the EDTA would chelate all the available calcium making it unavailable for analysis.

If possible take the sample at the same time as the PTH sample.

Collection technique

  1. Obtain a blood sample in a plain non-gel tube and fill to the brim with blood.
  2. Screw the lid down so no air enters the tube and allow to clot at room temperature for at least 30 minutes.
  3. Spin the sample and separate the serum into another plain tube, take care to leave as small an air gap as possible..
  4. Mark the tube with “Serum” and animal/owner name.
  5. If possible do not freeze the iCa sample, store in the fridge and send with the PTH sample outside the pack
  6. The laboratory will apply a correction formula to the result to take account of the changing pH that will have occurred due to the exposure to air. This will result in an estimated iCa at a standardised pH.