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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.
Hormone levels. See also: Hypothalamicâpituitaryâthyroid axis. TSH (with a half life of about an hour) stimulates the thyroid gland to secrete the hormone thyroxine (T4), which has only a slight effect on metabolism. T4 is converted to triiodothyronine (T3), which is the active hormone that stimulates metabolism. About 80% of this conversion is in the liver and other organs, and 20% in the thyroid itself. TSH is secreted throughout life but particularly reaches high levels during the periods of rapid growth and development.