Triiodothyronine is a thyroid hormone, one of the more important central regulators of metabolic function in mammals. Longer lived mammal species, such as naked mole rats, have markedly lower levels of this hormone. As it turns out, long-lived humans seem to have slightly lower levels of triiodothyronine as well: "The hypothalamo-pituitary-thyroid axis has been widely implicated in modulating the aging process. Life extension effects associated with low thyroid hormone levels have been reported in multiple animal models. In human populations, an association was observed between low thyroid function and longevity at old age, but the beneficial effects of low thyroid hormone metabolism at middle age remain elusive. ... We have compared serum thyroid hormone function parameters in a group of middle-aged offspring of long-living nonagenarian siblings and a control group of their partners, all participants of the Leiden Longevity Study. ... When compared with their partners, the group of offspring of nonagenarian siblings showed a trend toward higher serum thyrotropin levels [in] conjunction with lower free thyroxine levels [and] lower free triiodothyronine levels ... Compared with their partners, the group of offspring of nonagenarian siblings show a lower thyroidal sensitivity to thyrotropin. These findings suggest that the favorable role of low thyroid hormone metabolism on health and longevity in model organism is applicable to humans as well."