Diel variations in circulating levels of triiodothyronine and thyroxine in goldfish, Carassius auratus

Abstract

Goldfish maintained on a 12 h light: 12 h dark photoperiod (light onset 0800 hours) and sampled at one of six different times of day had significant differences in both serum triiodothyronine (T3) and thyroxine (T4) levels depending on the time of day of sampling. Hormone titers were determined by commercial radioimmunoassay kits. Maximum levels for T3 and T4 occurred at 1600 hours; minimum levels occurred during the 4 h preceding the onset of light (0400- and 0800-hour samples). There was no apparent effect of sampling stress (< 15 min) on titers of T3 and T4.