The thyroid gland uses iodine to produce thyroid hormones — primarily thyroxine (T4) and triiodothyronine (T3). T4 gets converted into T3 (a more active form) in the blood. Thyroid hormones regulate our metabolic rate and affect weight and energy level. The thyroid also produces calcitonin, which contributes to calcium balance. Thyroid hormone production is regulated by a feedback system involving the pituitary gland (a small gland at the base of the brain).