Radioimmunoassay of serum total thyroxine and triiodothyronine in healthy cats: assay methodology and effects of age, sex, breed, heredity and environment.

Double antibody radioimmunoassays for thyroxine (T4) and triiodothyronine (T3) were established for use in cats, using standards in T4-and T3-free, normal cat serum. The assay working ranges were 5–200 nmol/1 for T4 and 0.125-10 nmol/1 for T3. Recovery of added hormone was 106 per cent ± 5.9 for T4 and 104 per cent ± 8.5 for T3 (mean ± 1 s.d.). Mean between-assay coefficients of variation (c.v.) were 6.6 per cent for T4 and 12–1 per cent for T3. Serum total T4 levels of 318 and total T3 levels of 299 healthy cats aged 4 months to 13 years were measured. Mean ± 1 s.d. values were 26.1 ± 10.1 nmol/1 for T4 and 0.69 ± 0.29 nmol/1 for T3. T4 and T3 concentrations within individual animals were highly correlated. T4 levels in both sexes tended to decrease until approximately 5 years of age and then rise again. A similar, though less pronounced effect, was found for T3, concentrations for older cats levelling out rather than rising. For any given age, females and neutered females tended to have significantly higher T4 values than males and neutered males, but such effects were not significant for T3. When other effects were accounted for, the effects of neutering were not significant for levels of either hormone. Pedigree animals tended to have higher levels of T3 at any given age than domestic short- and long-haired cats taken as one group. Animals living in the same environment had significantly similar T4 levels and T3 levels. For T3, this appeared to be due to a definite genetic component, but it was not possible to differentiate between environmental and genetic effects for T4.