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

This study was to determine the daily fluctuation of serum thyroxine (tT(4)), free thyroxine (fT(4)), 3,5,3'-triiodothyronine (T(3)) 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 tT(4), fT(4), and T(3) concentrations were obtained at 3-hour intervals from 8 : 00 to 20 : 00. Serum tT(4), fT(4), and T(3) concentrations were measured by the enzyme chemiluminescent immunoassay (ECLIA). Mean T(3) concentrations had no significant differences according to the sample collection time during the day. Mean tT(4) and fT(4) concentrations at 11 : 00 were 3.28 ± 0.86 µg/dl and 1.30 ± 0.37 ng/dl, respectively and mean tT(4) and fT(4) at 14:00 were 3.54 ± 1.15 µg/dl and 1.35 ± 0.12 ng/dl, respectively. These concentrations were significantly high compared with tT(4) and fT(4) concentrations at 8:00, which were 1.75 ± 0.75 µg/dl and 0.97 ± 0.25 ng/dl, respectively (p < 0.05). According to the sample collection time, mean tT(4) and fT(4) concentrations changed with similar fluctuation during the day. Based on these results, it was considered that measurement of serum tT(4) and fT(4) concentrations from 11 : 00 to 14 : 00 might more easily diagnose the canine hypothyroidism in practice.