Serum T(4) and serum T(3) concentrations in immature captive whitetip reef sharks, Triaenodon obesus.

Serum T(3) (3,5,3' triiodothyronine) and serum T(4) (thyroxine) concentrations were repetitively assayed by radioimmunoassay over a three-year period in two male and two female immature captive whitetip reef sharks, Triaenodon obesus. These sharks were maintained at the Waikiki Aquarium, Honolulu, Hawaii, in an open system holding pool receiving 568 liters per minute of water from a saltwater well with an iodide concentration of 0.076 mg/liter. No significant male-female difference was observed for either serum T(3) or serum T(4). No seasonal pattern of serum T(3) was detected (P = 0.07). Serum T(3) concentrations ranged (mean +/- SEM) from 0. 52 to 0.83 ng/mL (0.67 +/- 0.01; n = 64). A significant seasonal difference was observed for serum T(4) (P < 0.001). Serum T(4) concentration was higher in winter (October-January) with a mean (range +/- SEM) of 6.58 ng/mL (1.48-8.77 +/- 0.35; n = 24) and lower in summer (May-August) with a mean of 3.62 ng/mL (1.34-5.71 +/- 0. 22; n = 24). The thyroid hormone T(4) has a seasonal rhythm even in immature sharks and may have an important role in physiology. J. Exp. Zool. 284:500-504, 1999.