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Hyperthyroidism is caused by usually bilateral (70%) adenomatous hyperplasia of the thyroid gland. Hyperthyroidism occurs in middle-aged to older (> 6 yr) cats; there is no breed or sex predilection. Hyperthyroidism occurs more frequently in indoor cats and those consuming a canned fish-based diet or diets containing liver flavor and giblets. The disease is common in certain coastal areas of the United States as well as in England and Australia, but is uncommon in Japan, France, and South Africa.
HISTORICAL AND PHYSICAL EXAMINATION FINDINGS
Historical findings include weight loss in conjunction with polyphagia, restlessness and increased activity, respiratory distress, weakness, vomiting, diarrhea, unkempt appearance or overgrooming, and such behavioral changes as aggression and inappropriate elimination. The most common physical examination finding is goiter (above image). Other physical examination findings may include tachycardia, panting, hyperthermia, anxiety and overactivity, mydriasis, and long toenails. On rare occasions, cardiac arrhythmia, gallop rhythm, or systolic murmurs may be found on auscultation. Common laboratory findings can include leukocytosis with eosinophilia, erythrocytosis, increased liver enzymes (alanine aminotransferase, aspartate aminotransferase, and alkaline phosphatase), and mild azotemia. Low serum cholesterol is an uncommon finding, and urinalysis is often unremarkable.
Basic Diagnosis
Hyperthyroidism can be diagnosed in approximately 90% of cats simply by measuring total serum concentrations of T4 (Table). However, this article focuses on the other 10%-those cats with occult hyperthyroidism that have clinical signs suggestive of hyperthyroidism (polyphagia, polydipsia, polyuria, weight loss, goiter) but exhibit normal serum total T4 concentrations at the time of examination. These cats may be challenging to diagnose for three reasons: 1) the hyperthyroidism may be mild or in the early stages, 2) the cat might have another nonthyroid illness that is suppressing total T4 into the normal range, and 3) T4 levels tend to fluctuate (on a daily basis or perhaps even more frequently).
Early or Mild Hyperthyroidism: Repeat the T4
Middle-aged and older hyperthyroid cats that present with weight loss and/or goiter but otherwise seem normal may be difficult to diagnose in the early stages of disease. Resting serum concentrations of both T4 and T3 are above the normal range in most cats with hyperthyroidism; however, some cats with hyperthyroidism (up to 10%) have serum concentrations of T4 that are within the mid- to high-normal range. Because many hyperthyroid cats with normal serum concentrations of T4 have relatively early hyperthyroidism, it is likely that the thyroid hormone concentrations will eventually increase into the thyrotoxic range if the disorder remains untreated.
Thyroid hormone concentrations in cats with mild hyperthyroidism may fluctuate into the normal range in some cats, suggesting that a diagnosis of hyperthyroidism cannot be excluded on the basis of the finding of a single normal to high-normal serum T4 or T3 result alone. In cats with clinical signs consistent with hyperthyroidism and in cats with palpable thyroid nodules, more than one serum T4 determination could be required to confirm a diagnosis. Therefore, the first step should always be to repeat the basal T4 measurement; the second serum T4 determination should be made at least 1 to 2 weeks later. An elevated serum T4 concentration is diagnostic of hyperthyroidism. However, a single serum T4 level in the normal range in a cat with hyperthyroid signs does not rule out the disease.
Because hyperthyroidism has become more common and recognized in its early stages, serum free T4 concentrations have recently been shown to be more diagnostic of early or "occult" hyperthyroidism. If the repeat T4 test is also equivocal, then free T4 is the next test indicated.
Circulating thyroid hormones can either be bound to carrier proteins or free (unbound) in the plasma. Most commercial T4 and T3 assays measure total concentrations, both free and protein-bound. Because only the free fraction of thyroid hormone is available for entry into the cells, free T4 determinations may provide a more consistent assessment of thyroid gland status than total T4 concentrations. The finding of a high free T4 concentration (despite mid- to high-normal levels of T4) in a cat with a consistent history (e.g., weight loss despite good appetite) and physical examination findings (e.g., palpable thyroid nodule) supports the diagnosis of early occult hyperthyroidism. However, measurement of free T4 levels does have the potential for false-positive results (see next section).