Thyroid function tests in healthy kittens aged between 2 and 16 weeks.

Objectives: This study aimed to determine thyroid hormone concentrations in a cohort of healthy kittens due to the paucity of information in the literature, and the potential for congenital hypothyroidism (CH) to contribute to fading kitten syndrome (FKS).

Methods: The serum concentrations of total thyroxine (TT4), free thyroxine (fT4), total triiodothyronine (TT3), free triiodothyronine (fT3) and thyroid-stimulating hormone (TSH) were measured in 19 healthy kittens aged 2-16 weeks.

Results: Mean TT4, fT4, TT3 and fT3 concentrations significantly differed across age groups. Mean TT4 and fT4 concentrations peaked at 6 and 5 weeks of age, respectively. The TT4 concentration exceeded the adult cat reference interval (ACRI) in 54% (32/59) of samples at week 6, with the highest TT4 concentration being 7.1 µg/dl (91 nmol/l). Mean TT3 and fT3 concentrations also peaked at 6 weeks of age. Mean TT3 concentration started below the ACRI until 4 weeks of age, after which it remained within the ACRI. The mean fT3 concentration was within the ACRI at all ages. The mean TSH concentration did not differ across age groups and remained within the ACRI in nearly 100% of samples.

Conclusions and relevance: Peak TT4, fT4, TT3 and fT3 concentrations in healthy kittens at 5 and 6 weeks of age are likely due to changes in the maternal transfer of thyroid hormones after weaning and organ system development. Knowing healthy neonatal and pediatric thyroid hormone concentrations in a cohort of kittens might help a veterinarian interpret thyroid hormone levels when trying to rule out CH in a kitten with FKS.