Thyroid hormones control metabolism, growth, body temperature, muscle strength, appetite, and the health of your heart, brain, kidneys, and reproductive system. Thyroid cells are the only cells in the body that absorb iodine. The thyroid uses iodine to create the T4 (thyroxine) and T3 (triiodothyronine) hormones. The thyroid responds to the levels of Thyroid Stimulating Hormone (TSH) secreted by the brain’s pituitary gland.