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Four tests â thyroid-stimulating hormone (TSH), Free T3, Free T4 and thyroid antibodies â are required to accurately assess the function of the thyroid gland, our master gland of metabolism. An optimal TSH level should be less than 2.0, not the currently accepted 4.7 reported by most labs.
Other determinants of resting metabolism include thyroid hormones and catecholamines, genetics, body temperature, and in women, the phase of the menstrual cycle. An increased release of thyroid hormones or catecholamines increases RMR. Conversely, a reduction in energy intake specifically decreases triiodothyronine (T3) levels and norepinephrine release, which can subsequently decrease metabolic rate.