is a glycoprotein hormone that forms in the anterior pituitary gland. It stimulates growth and thyroid function. The synthesis and release of thyroid hormones (T3 und T4) is controlled by TSH (thyroid stimulating hormone).
is the main hormone produced and released by the thyroid gland. T4 shows little activity and is classified as a prohormone. T4 is largely bound to protein, only a small percentage is unattached. It is worth mentioning that only free T4 (fT4) is active and serves as a precursor of T3.
is the more active thyroid hormone. It is 10 times stronger than T4. T3 is largely produced by T4 outside the thyroid.