The physiological relevance of brown adipose tissue (BAT) in humans is largely unknown. The investigators have shown that suppressing ovarian function in premenopausal women reduces resting energy expenditure (REE), and this is prevented by adding back estradiol (E2). The investigators preliminary data suggest that this may be due, in part, to reduced brown adipose tissue (BAT) activity. The overarching hypothesis is that BAT activity in humans is modulated by E2. To determine if natural declines in endogenous E2 contribute to changes in BAT activity, we will compare BAT activity in pre-and post-menopausal women. The investigators will also explore whether suppression of ovarian hormones in pre-menopausal women impairs BAT activity. BAT activity will be quantified using dynamic positron emission topography/computed tomography (PET/CT) imaging combined with 11C-acetate tracers. We will assess the thermogenic response of BAT by measuring cold-induced changes in REE, shivering, and skin and core temperature.