Thyroid hormone periodicity in healthy adult geldings

Summary Serum samples were collected from 10 healthy geldings every 4 h for three consecutive days and the triiodothyronine (T 3 ) and thyroxine (T 4 ) concentrations determined by radioimmunoassay. There were significant differences in the hormone concentrations related to time. The mean (± sd) T 3 concentration peaked around 08.00 h at a level (54.06 ± 14.02 ng/dl) significantly (P<.001) higher than the lowest concentration (38.71 ± 10.81 ng/dl) around midnight. Although the highest mean T 3 level was 08.00 h, this value was not significantly different from the noon and 16.00 h levels. Likewise, the mean T 3 level at midnight was not significantly different from the 20.00 h and 04.00 h levels, resulting in a plateau from 08.00 h to 16.00 h and a trough from 20.00 h to 04.00 h. The mean (± sd) T 4 concentration peaked around 16.00 h at a level (2.43 ± .81 μg/dl) significantly (P<.01) higher than the lowest concentration (1.79 ± .63 μg/dl) around 04.00 h.