Prospective evaluation of clinical, laboratory and scintigraphic features of naturally occurring hypothyroidism in 4 adult cats

Aim To screen laboratory submissions for cats with suspected spontaneous hypothyroidism.

Methods Submissions (geriatric profile, total thyroxine [TT4]) to a commercial laboratory between January 2022 and April 2023 were prospectively screened for cats with low TT4 aged 3-12 years. In these cats, TSH was measured. Iatrogenic hypothyroidism due to hyperthyroidism treatment was excluded by contacting submitting veterinarians. Creatinine and cholesterol concentration and red blood cell count (RBC) of cats with suspected spontaneous hypothyroidism (low TT4, TSH > 0.53 ng/ml [total allowable error of the upper reference range (RR)]) was compared to cats with TT4 within RR and cats with low TT4 but normal TSH (< 0.3 ng/ml; laboratory upper RR) by non-parametric tests. Data is presented as medians. Significant was P<0.05. Cats with suspected hypothyroidism were invited for thyroid scintigraphy and TSH stimulation test.

Results Overall, 31572 submissions of cats aged 3-12 years were included. 4492/31572 cats had low TT4, 1201/4492 had concurrently TSH > 0.3 ng/ml and 115/1201 had TSH > 0.53 ng/ml. Of those 115 cats, 62 were excluded due to hyperthyroidism treatment. Cats with suspected spontaneous hypothyroidism (n=53/31572; prevalence 0,17%) had higher creatinine (149 vs. 129 μmol/l, P=0.002) and lower RBC (7.9 vs. 9 x10¹²/l, P=0.0001) than age-matched cats with TT4 within RR (n=25274) as well as higher creatinine (149 vs. 122 μmol/l, P=0.004) than cats with low TT4 but normal TSH (n=3290). 23/41 (56%) cats with suspected hypothyroidism with known creatinine were azotaemic (creatinine > 140 µmol/L). There was no difference between groups for remaining parameters. 4/53 cats presented for further work-up and hypothyroidism was confirmed based on scintigraphy and TSH-stimulation test.

Conclusions Spontaneous hypothyroidism rarely occurs in cats. Increased creatinine might indicate the need for TSH testing in cats with low TT4, potentially suffering from spontaneous hypothyroidism. Hypercholesterolaemia is not an indicator of hypothyroidism in cats.

Publication History

Article published online:
07 March 2024