Thyroid hormones are essential for normal development, growth, and metabolic functions in
higher vertebrates. The thyroid gland produces and secretes mainly the prohormone L-
thyroxine (T 4), which is further metabolized by three deiodinase enzymes (5′ DI, 5′ DII,
and 5DIII) to yield thyromimetically active hormones and inactive metabolites or derivatives
with regulatory properties in thyroid hormone economy. The essential trace element iodine
is required for thyroid hormone synthesis, 1 and iodine contributes 65% of the mass of L …