Thyroid hormones and efficiency of energy utilization in mice selected for body weight.

Plasma thyroid hormones and energetic efficiency were examined in selected lines of mice during the fifth, sixth and seventh weeks of age. Mice from an unselected control line (C2) and from lines selected for large (H6) or small (L6) 6-week body weight were fed either ad libitum or restricted to approximately 75% of ad libitum intake. The H6 line was more efficient in energy deposition than the C2 and L6 lines with regression coefficients of change in body energy (kcal/kg0.75) on metabolizable energy (ME) intake (kcal/kg0.75) of 0.46+/-0.04, 0.26+/-0.03 and 0.26+/-0.03, respectively. Double-antibody radioimmunoassays for triiodothyronine (T3) and thyroxine (T4) were validated for unextracted mouse plasma. Plasma T4 concentration was greater (P less than 0.01) in the H6 line than in L6 and C2 lines, with the means of ad libitum mice being 36.7+/-1.6, 26.3+/-1.2 and 24.5+/-1.5 ng/ml, respectively. Plasma T3 concentration did not differ between lines but decreased (P less than 0.01) at 7 weeks of age. Feed restriction decreased (P less than 0.05) T4 concentration from 29.2+/-0.8 to 26.7+/-0.8 ng/ml and decreased (Pless than 0.01) T3 concentration from 1.93+/-0.02 to 1.82+/-0.02 ng/ml. Selection for 6-week body weight altered the efficiency of energy deposition and the thyroid hormone plasma concentration.