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Thyroid Hormones and Efficiency of Energy Utilization in Mice Selected for Body Weight^1,2,

Department of Animal Science, North Carolina State University, Raleigh, NC 27650

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/kg^0.75) on metabolizable energy (ME) intake (kcal/kg^0.75) 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 < 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 < 0.01) at 7 weeks of age. Feed restriction decreased (P < 0.05) T₄ concentration from 29.2±0.8 to 26.7±0.8 ng/ml and decreased (P < 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.

KEY WORDS: • thyroid hormone • T₃ • T₄ • mice • energetic efficiency • selected lines

¹ Paper No. 6127 of the Journal Series of the North Carolina Agricultural Research Service, Raleigh. The use of trade names in this publication does not imply endorsement by the North Carolina Agricultural Research Service of the products named nor criticism of similar ones not mentioned.

² Supported in part by NIH Grant No. 1 RO1 AM21950-01 NTN. Presented in part at the Annual Meetings of the Federation of American Societies for Experimental Biology, Dallas, TX, April 1979. Fed. Proc. 38, 554.

Manuscript received 12 September 1979.