Age-related differences in body weight loss in response to altered thyroidal status.

To determine whether age-related differences in body weight loss in hyperthyroidism could be related to caloric intake, the body weight and food consumption of Fischer 344 male rats were monitored every other day for four weeks. Six-month-old (young) rats were compared to 16-month-old rats (intermediate age) and 25-month-old (aged) rats. Hypothyroidism was induced with 0.025% methimazole in the drinking water for four weeks. Hyperthyroidism was induced with triiodothyronine (T3) injections (15 micrograms/100 g body weight i.p.) for the last 10 days of observation. A group of young rats pair fed with aged rats was included as a control group. The body weight changes of aged rats were similar to hypothyroid young rats. An index of T3 catabolic effect was calculated based on the net weight loss and food intake. This index was not different in aged rats compared to young rats. The apparent hypersensitivity of aged rats to T3 as evidenced by excessive weight loss could totally be attributed to decreased caloric intake. It is concluded that aged rats compared to the young are not more sensitive to the overall catabolic effects of thyroid hormones.