Iodine is an essential
element for
thyroid hormone synthesis. The thyroid gland has the capacity and holds the machinery to handle the
iodine efficiently when the availability of
iodine becomes scarce, as well as when
iodine is available in excessive quantities. The latter situation is handled by the thyroid by acutely inhibiting the organification of
iodine, the so-called acute Wolff-Chaikoff effect, by a mechanism not well understood 52 years after the original description. It is proposed that iodopeptide(s) are formed that temporarily inhibit
thyroid peroxidase (TPO)
mRNA and
protein synthesis and, therefore,
thyroglobulin iodinations. The Wolff-Chaikoff effect is an effective means of rejecting the large quantities of
iodide and therefore preventing the thyroid from synthesizing large quantities of
thyroid hormones. The acute Wolff-Chaikoff effect lasts for few a days and then, through the so-called "escape" phenomenon, the organification of intrathyroidal
iodide resumes and the normal synthesis of
thyroxine (T4) and
triiodothyronine (T3) returns. This is achieved by decreasing the intrathyroidal inorganic
iodine concentration by down regulation of the
sodium iodine symporter (NIS) and therefore permits the TPO-H202 system to resume normal activity. However, in a few apparently normal individuals, in newborns and fetuses, in some patients with chronic systemic diseases, euthyroid patients with
autoimmune thyroiditis, and
Graves' disease patients previously treated with radioimmunoassay (RAI), surgery or
antithyroid drugs, the escape from the inhibitory effect of large doses of
iodides is not achieved and clinical or subclinical
hypothyroidism ensues.
Iodide-induced
hypothyroidism has also been observed in patients with a history of
postpartum thyroiditis, in euthyroid patients after a previous episode of
subacute thyroiditis, and in patients treated with recombinant
interferon-alpha who developed transient thyroid dysfunction during
interferon-a treatment. The
hypothyroidism is transient and thyroid function returns to normal in 2 to 3 weeks after
iodide withdrawal, but transient T4 replacement
therapy may be required in some patients. The patients who develop transient
iodine-induced
hypothyroidism must be followed long term thereafter because many will develop permanent
primary hypothyroidism.