Thyroid hormone periodicity in healthy adult geldings

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Abstract
Serum samples were collected from 10 healthy geldings every 4 h for three consecutive days and the triiodothyronine (T3) and thyroxine (T4) concentrations determined by radioimmunoassay. There were significant differences in the hormone concentrations related to time. The mean (.+-. sd) T3 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 T3 level was 08.00 h, this value was not significantly different from the noon and 16.00 h levels. Likewise, the mean T3 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) T4 concentration peaked around 16.00 h at a level (2.43 .+-. .81 .mu.g/dl) significantly (P < .01) higher than the lowest concentration (1.79 .+-. .63 .mu.g/dl) around 04.00 h.