title: Thyroid hormone periodicity in healthy adult geldings.
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 less than .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 micrograms/dl) significantly (P less than .01) higher than the lowest concentration (1.79 +/- .63 micrograms/dl) around 04.00 h.
authors.affiliation: Department of Food Animal and Equine Medicine, School of Veterinary Medicine, North Carolina State University, Raleigh 27606.
authors.forename: W M
authors.initials: WM
authors.lastname: Duckett
authors.forename: J P
authors.initials: JP
authors.lastname: Manning
authors.forename: P G
authors.initials: PG
authors.lastname: Weston
mesh_terms: D000818:Animals; D002940:Circadian Rhythm; D006736:Horses / Q000097:blood*; D008297:Male; D011863:Radioimmunoassay; D012016:Reference Values; D013974:Thyroxine / Q000097:blood*; D014284:Triiodothyronine / Q000097:blood*
chemical_list: D014284:Triiodothyronine; D013974:Thyroxine