iThei mdcdaelcinfd o c elllllrlllll tThe hlcinfd olsbdkpvouvbody through an oxidation process. The enzyme thyroid peroxidase
(TPO) converts iodide to the active iodine, making hydrogen per-
oxide as a byproduct. Now ready to bind with other molecules, the
reactive iodine attaches to the amino acid tyrosine in thyroglobulin
through a process known as “iodination.”
During iodination, each molecule of tyrosine combines with one or
two iodine molecules, resulting in either monoiodotyrosine (T1) or
diiodotyrosine (T2). The molecules then bond to form either tri-
iodothyronine (T3, thyroglobulin with three iodine molecules) or
thyroxine (T4, thyroglobulin with four iodine molecules).
T1+T2=T3 or T2+T2=T
Of the four iodinated molecules, only T3 and T4 are biologically ac-
tive in the body. Thyroxine (T4) is known as a prohormone, however,
and is 300 percent less biologically active than T3. Triiodothyronine
(T3) is the main biologically active thyroid hormone. These mol-
ecules are stored in thyroid follicles until they are needed.
Twenty percent of the T3 hormone comes from thyroid secretion,
while the remaining 80 percent comes from T4 when T4 is con-
verted to T3 through the deiodination process (which removes one
iodine molecule) in peripheral organs like the liver and kidney. Zinc
is required to convert T4 to T3.
Low levels of T3 and T4 signal release of TSH (thyroid stimulating
hormone), while high levels of circulating T3 and T4 stop the release
of TSH. In people with normal thyroid function, TSH levels may
fluctuate in times when more thyroid hormone is consumed, such
as in stress, illness, lack of sleep, pregnancy, or low temperatures.^15
Thyroid Hormone Disorders
Thyroid hormone disorders can be classified as those resulting in
inadequate thyroid hormone production (hypothyroidism) and