Circadian variations of serum thyroxine, free thyroxine and 3,5,3'triiodothyronine concentrations in healthy dogs

ABSTRACT

This study was to determine the daily fluctuation of serum thyroxine (tT4), free thyroxine (fT4), 3,5,3'-triiodothyronine (T3) concentrations in healthy dogs. Thyroid function of these dogs was evaluated on the basis of results of TSH response test. Samples for the measurement of serum tT4, fT4, and T3 concentrations were obtained at 3- hour intervals from 8 00 to 20 00. Serum tT4, fT4, and T3 concentrations were measured by the enzyme chemiluminescent immunoassay (ECLIA). Mean T3 concentrations had no significant differences according to the sample collection time during the day. Mean tT4 and fT4 concentrations at 11 00 were 3.28 +/- 0.86 microgram/dl and 1.30 +/- 0.37 ng/dl, respectively and mean tT4 and fT4 at 1400 were 3.54 +/- 1.15 microgram/dl and 1.35 +/- 0.12 ng/dl, respectively. These concentrations were significantly high compared with tT4 and fT4 concentrations at 800, which were 1.75 +/- 0.75 microgram/dl and 0.97 +/- 0.25 ng/dl, respectively (p < 0.05). According to the sample collection time, mean tT4 and fT4 concentrations changed with similar fluctuation during the day. Based on these results, it was considered that measurement of serum tT4 and fT4 concentrations from 11 00 to 14 00 might more easily diagnose the canine hypothyroidism in practice.