Choosing the best tests to diagnose feline hyperthyroidism - Veterinary Medicine
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Choosing the best tests to diagnose feline hyperthyroidism
Not all hyperthyroid cats exhibit clinical signs, and concurrent illness may skew the laboratory assessment of thyroid function. Here's the latest research on what tests to perform to establish a diagnosis.


VETERINARY MEDICINE


One of the challenges in diagnosing hyperthyroidism is the effect that nonthyroidal illness has on thyroid function tests in cats with concurrent hyperthyroidism. In a large study, researchers measured total T4, free T4 (by equilibrium dialysis), and total T3 concentrations in 917 cats with hyperthyroidism, 221 cats with nonthyroidal illness, and 172 clinically normal cats.5 They found that measuring free T4 by equilibrium dialysis was significantly more sensitive than measuring total T4 as a diagnostic test for hyperthyroidism. However, of the 221 cats with nonthyroidal illness, 12 cats (5.4%) had a high free T4 concentration measured by equilibrium dialysis (a false positive result). Therefore, the calculated specificity of measuring free T4 by equilibrium dialysis as a diagnostic test for hyperthyroidism was significantly lower than the specificity of measuring total T4. This study suggests that measuring free T4 by equilibrium dialysis is only indicated in those cats with clinical signs and a total T4 concentration in the upper 50% of the normal resting range. In these animals, the free T4 concentration will be elevated. In our practice, we see this most often in cats with nonthyroidal illnesses such as inflammatory bowel disease, gastrointestinal lymphoma, diabetes, and renal disease as well as in cats receiving glucocorticoids. The biggest challenge is deciding how to treat such cats. You must decide what role both diseases are playing with respect to the clinical signs and address each disease separately.

ALP, ALT, Calcium, and PTH

We know that many cats with hyperthyroidism have elevated alkaline phosphatase (ALP) or alanine transaminase (ALT) activities. Results of serum bile acids testing and histopathologic examination of liver biopsy samples in these cats are abnormal, but no clinical signs are present, and the abnormalities resolve after treating the hyperthyroidism in most cats. Hypercalcemia can also be seen in hyperthyroid cats. The calcium abnormalities appear to self-correct after successful treatment, and no other cause for the hypercalcemia has been found. Several recent studies have examined the effects of hyperthyroidism on bone and calcium metabolism in hyperthyroid cats.

Enzymes that arise from the liver, bone, the intestines, and other tissues constitute serum ALP. The source of elevated total serum ALP activity in hyperthyroid cats has been suggested to be from the liver and bone isoenzymes. In one study, ALP isoenzymes were measured by serum electrophoresis in five normal cats and in 34 hyperthyroid cats.6 Electrophoresis was used to identify and quantify ALP isoenzymes in homogenates of the kidneys, the intestines, bone, and the liver. In both groups, the highest concentration of ALP was found in the kidneys, followed by the intestines, the liver, and then bone. The liver isoenzyme of ALP was the only type identified in the serum of normal cats. Serum total ALP activity was elevated in 53% of hyperthyroid cats. Thirty of the 34 hyperthyroid cats had two main electrophoresis bands corresponding to liver and bone isoenzymes of ALP. A significant correlation was noted between the total T4 concentration and total serum ALP activity and the total T4 concentration and liver isoenzyme activity, but not between the T4 concentration and bone isoenzyme activity in hyperthyroid cats. The proportion of bone and liver isoenzymes of ALP in sera of hyperthyroid cats varied considerably, with the liver isoenzyme making up 17% to 100% of the total serum ALP and the bone isoenzyme constituting 0% to 74% of ALP. The researchers concluded that abnormalities in bone and liver isoenzymes account for the elevations in serum ALP found in hyperthyroid cats. This study points out that practitioners should evaluate thyroid function in older cats with increased serum ALP activities. Although bone disease has not been reported to occur in cats with hyperthyroidism, it is likely that the high bone ALP activity is related to increased bone turnover. This theory was reinforced in two studies evaluating bone turnover and calcium metabolism in hyperthyroid cats.7,8


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Source: VETERINARY MEDICINE,
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