The thyroid produces three hormones that affect the expression and function of other hormones in the body. Thyroid hormones are known as T3, T4, and Calcitonin. They are formed in the thyroid but need iodine (gleaned from diet) to develop fully. T3 and T4 control metabolism -- the "engine" -- with help from the pituitary gland, which signals how much T3 and T4 to release into the bloodstream.

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