Adult-onset spontaneous feline hypothyroidism (SH) is considered rare, but its prevalence is unknown. This study aimed to screen laboratory submissions for cats with laboratory suspected SH (LSSH) and to identify laboratory abnormalities associated with LSSH. Submissions to a commercial laboratory in Germany between January 2022 and April 2023 were prospectively screened for cats aged 3–12 years with low total thyroxine (TT4); in these cats, thyrotropin (TSH) was measured. Iatrogenic hypothyroidism was excluded by contacting submitting veterinarians. Creatinine, triglyceride and cholesterol concentration and red blood cell count (RBC) of cats with LSSH (low TT4, TSH > 0.53 ng/mL; i.e., [one sided TSH reference change value (76%) of TSH upper reference interval (RI)] + [TSH upper RI] = 0.53) were compared to euthyroid cats (TT4 within RI) and cats with suspected non-thyroidal illness (SNTIS) (low TT4, normal TSH (<0.3 ng/mL; upper RI)) by non-parametric tests. p < 0.05 was significant. In total, 31,572 submissions of cats were included, 25,169 (79.7%) were euthyroid, 3818 (12.1%) had SNTIS and 61 (0.2%) had LSSH. Cats with LSSH had higher creatinine (p = 0.002) and lower RBC count (p < 0.0001) than euthyroid cats as well as higher creatinine (p = 0.00035) than cats with SNTIS. Azotaemia (creatinine > 140 µmol/L) and anaemia (RBC < 7.2 × 10¹²/L) were present in 28/49 (57%) and 19/44 (43%) cats with LSSH, respectively. There was no difference between LSSH and SNTIS for the remaining parameters. In cats with low TT4, azotaemia and anaemia might indicate the presence of SH and reinforce the need for TSH testing. Hypercholesterolaemia and hypertriglyceridaemia are not indicators of SH.