Utilization rates of thyroid hormones in mammals

1. Thyroxine utilization rates (T4U: N = 37 species) and triiodothyronine utilization rates (T3U: N = 7 species) scale with body mass to the 0.81 and 0.74 power respectively. 2. T4U rates tend to be lower in summer relative to other seasons, vary unpredictably during pregnancy and lactation, increase with regular physical activity, and generally decrease with age. 3. Both T4U and T3U increase with cold exposure, decrease with heat exposure and during fasting, and increase/decrease with hyperthyroidism/hypothyroidism. 4. Since these T4U and T3U changes are qualitatively similar but quantitatively different, the T3U/T4U ratio varies, suggesting a variable deiodination rate from thyroxine to triiodothyronine.