Congenital hypothyroidism (CH) is associated with significant morbidity and mortality in pediatric felines. Thyroid hormones (TH) are essential for normal development of the nervous and skeletal systems. The paucity of information on TH levels and the inability to identify early histopathologic changes has led to CH underdiagnosis. The literature on TH levels in kittens is sparse and incongruent, comprised of mostly case reports. References state kitten total thyroxine (TT4) levels are 2 to 3 times higher than the adult cat TT4 level. They also make the assumption free thyroxine (fT4) levels are higher than the adult cat. An abstract was previously presented showing kitten TH levels did not exceed the adult cat normals. These values were measured with assays that are no longer used. The normal pediatric TH levels must be evaluated with the newer and currently available assays. The purpose of this study was to determine the TT4, fT4, total tri-iodothyronine (T3), free tri-iodothyronine (fT3), and thyroid stimulating hormone (TSH) levels in healthy kittens aged 2 through 16 weeks measuring levels at weekly intervals using the current available diagnostic methods.
Normal, healthy kittens from a research colony in an approved facility at the University of Pennsylvania, School of Veterinary Medicine were used. One half to one milliliter of whole blood based on body weight was drawn by jugular venipuncture each week. Serum was immediately obtained and frozen until analysis. The samples were submitted to the Michigan State University Diagnostic Laboratory for Animal and Population Health. All TH measurements were performed on serum. The TSH was measured using a canine chemiluminescent assay. The TT4, TT3 and fT3 were measured with radioimmunoassays and the fT4 was measured using equilibrium dialysis. All assays have been previously validated in cats.
Samples were collected at the same time in the morning at weekly intervals. All values are reported as the mean. The TT4 was initially above the adult cat reference range (ACRR; 10–47 nmol/L) at 2 weeks and peaked at 6 weeks (50.9 nmol/L and 64.6 nmol/L, respectively). The TT4 returned to within the ACRR at 8 weeks, was elevated again at 10 weeks (50.0 nmol/L), returned within the ACRR at 13 weeks and remained stable through 16 weeks. The fT4 remained within the ACRR (10–53 pmol/L) through all 16 weeks, but showed a similar peak at 6 and 10 weeks (50 pmol/L and 39.8 pmol/L, respectively). The TSH was within the ACRR (0–0.38 ng/ml) throughout the 16 weeks. The TSH remained stable at 0.16 ng/ml from 2 through 4 weeks, decreased within the ACRR at 8 weeks, peaked at 10 weeks (0.18 ng/ml) and then continuously decreased to 0.06 ng/ml at 16 weeks. The TT3 and fT3 remained within the ACRR through all 16 weeks. Both hormones peaked at 6 weeks and remained at that level through 16 weeks.
The TH levels in pediatric kittens were measured with the newer assays and with the exception of TT4, all TH levels were within the ACRR. The TT4 did not elevate more than twice the upper limit of the ACRR at any time point and was consistently within the ACRR after 12 weeks of age. A peak in TT4, fT4, TT3 and fT3 was seen at 6 weeks and a peak in TT4 and fT4 was also seen at 10 weeks. TSH levels using the canine TSH assay have been validated for use in adult cats, but these values have not yet been reported in kittens. The TSH levels remained within the ACRR and it showed a similar peak at 10 weeks. These patterns likely occur due to an increased growth and development at 6 and 10 weeks.