Enigmas of the Thyroid Gland

VRATISLAV SCHREIBER, MD, DrSc Prof. Clin. Physiol., Lab. for Endocrinology & Metab., IIIrd Med. Dept., lst Sch. of Med., Charles University (General University Hospital) Prague 128 21, Czech Republic Motto: To whom everything is clear has one sorrow less (J. Charvát) Introduction Enigma No. 1.: The thyroid gland is (or was) a part of the gastrointestinal system Enigma No. 2.: Thyroxine is a prohormone Enigma No. 3.: Why do T4 and T3 act as steroid hormones? Enigma No. 4.: Thyroid hormones induce catabolism by enhancing proteoanabolism Enigma No. 5.: The thyroid gland is immunologically privileged Enigma No. 6.: Mutations of the TSH receptor Enigma No. 7.: Dependence of the thyroid gland on environment (iodine intake) The magic number 7 References