3 yo MN cat for blood loss anemia. Chem panel shows hypoalbuminemia and total serum calcium mildly decreased. What do you expect ionized calcium and PTH to be?

3 yo MN cat for blood loss anemia. Chem panel shows hypoalbuminemia and total serum calcium mildly decreased. What do you expect ionized calcium and PTH to be?

• A. Normal ionized calcium, normal PTH.
• B. Low ionized calcium, high PTH.
• C. High ionized calcium, low PTH.
• D. Low ionized calcium, low PTH.

Answer: (A)

The key concept is that ionized calcium is the physiologically active form, while total calcium can be influenced by albumin levels. Albumin binds calcium, so when albumin is low (hypoalbuminemia), total calcium tends to fall even if the amount of free (ionized) calcium is unchanged. Parathyroid hormone (PTH) responds to the ionized calcium concentration, not the total calcium. In this cat with blood loss anemia, hypoalbuminemia explains a mildly decreased total calcium, but ionized calcium would remain normal. Because the ionized calcium is normal, there’s no trigger for PTH to rise or fall, so PTH would also be normal. Hence, the expected values are: normal ionized calcium and normal PTH.

The key concept is that ionized calcium is the physiologically active form, while total calcium can be influenced by albumin levels. Albumin binds calcium, so when albumin is low (hypoalbuminemia), total calcium tends to fall even if the amount of free (ionized) calcium is unchanged. Parathyroid hormone (PTH) responds to the ionized calcium concentration, not the total calcium.

In this cat with blood loss anemia, hypoalbuminemia explains a mildly decreased total calcium, but ionized calcium would remain normal. Because the ionized calcium is normal, there’s no trigger for PTH to rise or fall, so PTH would also be normal. Hence, the expected values are: normal ionized calcium and normal PTH.