Screening for naturally occurring hypothyroidism in adult cats: A prospective multi-center study in Central Europe.

Introduction: Research on naturally occurring hypothyroidism in adult cats is limited to 16 reported cases in the last three decades, highlighting the need for more comprehensive studies. This prospective multicenter study aimed to identify cases of spontaneous primary hypothyroidism (SPH) in adult cats presented over a one-year period . Staff members of two university hospitals and one private referral clinic screened for cats aged older than one year showing clinical signs suggestive of feline hypothyroidism including obstipation, obesity/weight gain, lethargy, goiter size, skin changes or mild azotemia. They were instructed to proceed with endocrine investigations if low or low-normal total T4 (TT4) concentrations were detected, using in-house analyses at two locations and external analyses at one university hospital. The continuative diagnostic investigations consisted primarily of thyroid-stimulating hormone (TSH) measurements using leftover samples and TT4 reevaluation. Additional cats with possible SPH were recruited by screening the local documentation systems for low to low-normal TT4 concentrations recorded during the study period. Low (n = 253, 18,3 %) to low-normal (n = 377, 27,3 %) TT4 concentrations were observed in 630 (45,6 %) of 1382 cats. Of these 630 possible SPH cats, 566 (89,8 %) cats were excluded for the following reasons: TT4 was measured to rule out hyperthyroidism (299 cats, 47,4 %), they were treated with drugs that lower TT4 concentrations (anti-thyroid medications: 155 cats, 24,6 %; other drugs: 95 cats, 15,1 %), or follow-up investigations could not be conducted (17 cats, 2,7 %). In 64 (10,2 %) cats of the 630 cats with low or low-normal TT4 concentration, SPH was considered a realistic differential diagnosis based on clinical signs and further endocrine investigations were recommended. SPH was excluded by TSH (n=55) or repeated TT4-measurements (n=8). Hypothyroidism was diagnosed in one cat with palpable bilateral goiter and suspected congenital dyshormonogenesis. Results suggest that SPH remains rare and further endocrine investigations e.g. TSH measurement are needed to find additional cases without any clinical suspicion.