Differences in thyroid activity of several strains of mice and F1 hybrids.

Thyroid secretion rate was determined in four strains of inbred mice and two groups of F1 hybrids. In all genotypes the thyroid secretion rate was consistently but not significantly greater in males than in females. The average thyroid secretion rates, expressed as micrograms l-thyroxine/ 100 gm body weight with the standard errors for the males and females, were: A/Jax 2.13 ± 0.12 and 1.84 ± 0.10; BALB/c 2.44 ± 0.21 and 1.84 ± 0.12; C57BR/cd 3.35 ± 0.34 and 2.45 ± 0.11; C57BL/6 4.19 ± 0.64 µg. and 3.33 ± 0.34. The secretion rate for hybrids CAF1 and BBF1 males and females, respectively, were: 2.34 ± 0.185 and 1.84 ± 0.106; 3.79 ± 0.644 and 2.74 ± 0.268 µg/100 gm. C57BL strain and BBF1 hybrid had significantly higher secretion rates than all other groups. The results are consistent with the output rate values reported earlier by the same authors, and clearly indicate that thyroid activity is under genetic control.