Early treatment of obese (ob/ob) mice with triiodothyronine increases oxygen consumption and temperature and decreases body fat content.

An early abnormality in the ob/ob mouse is a low circulating level of the thyroid hormone, triiodothyronine (T3). The possibility was explored that early T3 treatment of ob/ob mice will increase oxidative metabolism and lower body fat content. Doses of T3, ranging from 0.0 to 25.0 micrograms/100 g body wt were injected, ip, into ob/ob and non-ob/ob mice daily from 3 weeks until 6 weeks of age. Food intake was equalized across all groups to that consumed by non-ob/ob saline-treated group. At 6 weeks of age, body weight, serum concentrations of thyroxine (T4), T3, insulin and glucose, oxygen consumption, colonic temperature, and body composition were analyzed. T3 treatment decreased body weight, increased body oxygen consumption, increased colonic temperature, and decreased body fat without a significant change in body protein in ob/ob mice. T3 treatment also increased serum T3, and decreased serum T4, insulin, and glucose concentrations in ob/ob mice. Because total body protein did not change as a result of T3 treatment, the increased oxidative metabolism due to T3 treatment was probably via the change of metabolic activity of the total lean body mass or the specific metabolically active tissues in the ob/ob mice, such as brown adipose tissue, liver, or muscle.