LAUSR

Background Naturally occurring hypothyroidism in adult cats is rare, with only 4 cases reported. Objectives To describe the historical, clinical, laboratory, and scintigraphic features of adult cats with spontaneous hypothyroidism. Animals Seven adult cats referred for suspected hypothyroidism. Methods Prospective case series. We collected data on cats’ signalment, clinical signs, results of physical examination, routine laboratory and thyroid hormone testing, and thyroid imaging (thyroid scintigraphy or ultrasound). We subsequently treated cats with levothyroxine and evaluated their response to treatment. Results Cats ranged from 3.5 to 11 years, with no apparent breed predilection; 6/7 cats were male. Only 2/7 cats were initially tested because of signs of hypothyroidism (hair‐coat changes, lethargy, obesity); others were tested for routine thyroid monitoring or palpable thyroid nodules. Four were azotemic (serum creatinine, 2.2‐3.4 mg/dL). Six of the cats had low serum thyroxine (T4) and free T4 (fT4) concentrations, whereas all 7 cats had high thyroid‐stimulating hormone (TSH) concentrations. In 6/7 cats, thyroid scintigraphy revealed bilateral goiter with intense radionuclide uptake; imaging showed no visible thyroid tissue in the other. After levothyroxine treatment, serum concentrations of T4 and fT4 increased and TSH fell; high serum creatinine normalized in azotemic cats; and repeat imaging showed reduction in goiter size. Conclusions and Clinical Importance Primary hypothyroidism develops in adult cats, with a higher prevalence than previously thought. Most cats appear to develop a goitrous form of hypothyroidism associated with thyroid hyperplasia, whereas thyroid atrophy appears to be less common. With levothyroxine replacement, clinical and laboratory abnormalities improve or resolve.