Diagnostic Testing for Feline Thyroid Disease: Hypothyroidism

Although naturally occurring hypothyroidism is very rare in cats, iatrogenic hypothyroidism is a recognized complication of treatment for hyperthyroidism. However, confirming the diagnosis of hypothyroidism in cats is not generally straightforward. The potential for false-negative and false-positive results exists with all thyroid function tests, especially in older cats that may have concurrent nonthyroidal illness. Therefore, all thyroid function test results must be interpreted in light of the cat’s history, clinical signs, and other laboratory findings. If a low to low-normal serum thyroxine (T4) value is found in a cat that has been treated for hyperthyroidism, repeating the total T4 analysis, determining free T4 and thyroid stimulating hormone (TSH) concentrations, or performing a TSH stimulation test or thyroid scintigraphy may be needed to confirm the diagnosis. Diagnostic Testing for Feline Thyroid Disease: Hypothyroidism In cats, as in other species, hypothyroidism is the clinical syndrome that results from the chronic deficient secretion of the two thyroid hormones: thyroxine (T4) and triiodothyronine (T3). Unlike dogs, in which primary hypothyroidism is common, naturally occurring hypothyroidism is extremely rare in adult cats, with only two documented cases reported.5,6 Most commonly, feline hypothyroidism is an iatrogenic complication associated with overtreatment of hyperthyroidism. Spontaneous hypothyroidism, when it does develop in cats, is seen most commonly as a congenital form in dwarf kittens.1–4 Although many of the clinical features that develop in hypothyroid cats are similar to those seen in hypothyroid dogs, some major differences can make diagnosis more difficult in cats (TABLE 1).1–4 First, hypothyroid cats may develop a poor appetite, a sign not reported in dogs with hypothyroidism. Second, hypothyroid cats rarely develop severe hair loss or total alopecia, relatively common signs in dogs. The major clinical signs of hypothyroidism in adult cats are not specific but can include lethargy, decreased appetite, and weight gain (TABLE 1).1–4 Nonspecific cutaneous changes (e.g., nonpruritic seborrhea sicca, dull haircoat, excessive shedding or matting of hair) and obesity can develop, whereas hypothermia, and bradycardia are less common.1–4 Iatrogenic hypothyroidism can develop during therapy with antithyroid drugs,7–9 after thyroidectomy,7,9,10 or following radioiodine therapy.7,11–14 Although early reports suggested that clinical signs associated with severe iatrogenic hypothyroidism in cats were uncommon and that most cats did not require treatment, it is now realized that milder degrees of iatrogenic hypothyroidism are relatively common and that these cats may benefit from thyroid replacement therapy. Many cats with mild or subclinical hypothyroidism fail to develop noticeable clinical features (TABLE 1). A major potential benefit of treating iatrogenic hypothyroidism in cats is helping maintain renal function. It is well known that For more information, please see the companion article, “Diagnostic Testing for Feline Thyroid Disease: Hyperthyroidism.” Table 1. Clinical Features of Congenital, Iatrogenic, and Spontaneous Adult-Onset Hypothyroidism in Cats Lethargy Congenital Iatrogenic Adult Onset