L-Thyroxine secretion rates and l-triiodothyronine equivalents in the opossum ( Didelphis virginianus)

Abstract

One hundred and eleven determinations of thyroid secretion rate (TSR) were made on 57 opossums trapped in Boone County, Missouri, from February 1961 to May 1964. It was found that the adult population had a mean TSR of 1.72 μg l-thyroxine ( l-T 4)/100 gm b.w., and the juveniles (under 1 year of age) had a mean TSR of 2.66 μg T 4/100 gm b.w. The adult population exhibited a seasonal pattern of thyroid activity. In spring, the mean adult TSR was 1.94 ± .37 μg/100 gm body weight (b.w.), summer 1.19 ± .24 μg/100 gm b.w., fall 1.33 ± .47 μg/100 gm b.w. and winter 1.92 ± .47 μg/100 gm b.w The juvenile animals had a mean TSR of 2.84 ± .72 μg/100 gm b.w. in summer, 2.04 ± 1.08 μg/100 gm b.w. in fall, and 3.58 ± 1.90 μg/100 gm b.w. in winter, but seasonal variation was not significant. Differences between male and female TSR's within populations were not statistically significant. l-Triiodothyronine ( l-T 3) was substituted in the TSR procedure for l-T 4, and found to be approximately 1.1 times as effective as l-T 4 in blocking thyroidal I 131 release.