EXPRESS: Spontaneous adult-onset primary hypothyroidism in 17 cats.

ObjectivesThe aim of this study was to describe the clinical, laboratory, and thyroid imaging features of 17 adult cats with spontaneous hypothyroidism diagnosed over a 4-year period in four countries.MethodsThis multicenter case series included cats diagnosed with spontaneous hypothyroidism between 2021 and 2025. We collected data on cats' signalment, clinical signs, results of physical examination, routine laboratory and thyroid hormone testing, and thyroid imaging (scintigraphy, ultrasonography, or computed tomography). We also reviewed treatment data, including levothyroxine dosage, changes in serum thyroid hormone and creatinine concentrations, clinical progression, and follow-up scintigraphy.ResultsCats ranged in age from 2 to 14 years, with no breed predilection; 10 (58.8%) were male and 7 (41.2%) female. Common clinical signs included lethargy, dermatologic signs (unkempt haircoat, increased shedding, dandruff, hair thinning), weight gain or obesity, and palpable goiter. The most common laboratory findings included azotemia (5 cats), anemia (3 cats), and worsening of pre-existing diabetes mellitus (2 cats). Thirteen (76.5%) cats had low serum thyroxine (T4), whereas all cats had high thyroid-stimulating hormone (TSH) concentrations. Thyroid imaging confirmed undetectable or atrophic thyroid tissue in 10 cats (58.8%) and bilateral thyroid enlargement in 7 (41.2%). After levothyroxine treatment (median, 133 µg/cat/day) in 15 cats, serum total T4 concentration increased and TSH fell. Treated cats became more alert and active, goiter resolved, azotemia resolved or improved, and 2 achieved remission of concurrent diabetes mellitus.Conclusions and relevanceThis study broadens current knowledge of spontaneous hypothyroidism in adult cats, a condition that remains underrecognized in clinical practice. Atrophic hypothyroidism appears more common than previously recognized, although goitrous hypothyroidism was identified in >40% of cats. Spontaneous hypothyroidism should be considered in cats with unexplained azotemia and in diabetic cats with worsening glycemic control. Levothyroxine supplementation reliably improved or resolved clinical signs in all treated cats.