THYROID ACTIVITY IN LINES OF MICE SELECTED FOR LARGE AND SMALL BODY WEIGHT

Abstract

Thyroid activity was measured at 21, 31 and 42 days of age in lines of mice selected for large (H6) and small (L6) 6-week body weight and an unselected control (C2). Mean %181I uptake/(body weight).75was greatest at 21 days (weaning) and decreased exponentially thereafter with females generally having higher values than males. The L6line had a significantly (P < 0.01) higher %131I uptake/(body weight).75than H6at 31 and 42 days. For %131I uptake, disregarding body size, the H6line was significantly (P < 0.01) higher than C2, but the L6and C2, lines did not differ. Mean %131I turnover/day/(body weight).75showed results similar to %131I uptake per unit metabolic body size. However, when body size was not considered, %131I turnover in the L6line was slightly higher than H6at 31 and 42 days of age. Rate of maturation, as measured by age at opening of eyes and vagina and appearance of external ears, coat hair and nipples, was significantly (P < 0.01) more rapid in the H6line compared to L6. Within lines the correlation between growth rate and maturation rate was positive. These data suggest that there is a small positive genetic correlation between body weight at six weeks of age and thyroid activity in mice.