Iodine Deficiency – The Effects on the Thyroid
Depletion of thyroidal iodine.
With iodine deficiency in the diet, there is less iodine in the blood. To a certain extent, the thyroid can adapt by becoming more efficient. However, when the deficiency is long enough and severe enough, the amount of iodide in the thyroid becomes depleted.
Decrease in serum T4.
With a decrease in the iodide available in the thyroid, less T4 is created and secreted into the blood. Therefore, the serum T4 levels decrease.
Increase in serum TSH.
When the serum T4 levels fall, the pituitary responds by secreting more TSH (Thyroid Stimulating Hormone). (See Diagram). Normally, this increase in TSH would stimulate increased production of T4. But, because of the iodide deficiency, the thyroid does not have enough iodide to create more T4.
One of the effects of TSH is to increase the size of the thyroid. Moreover, low intrathyroidal iodide itself may cause thyroid growth.
Decrease in the degree of iodination of Tg.
With less iodine available in the thyroid, there is a decrease in the degree of iodination of the thyroglobulin (Tg).
Increase in thyroidal T1/T2 ratio.
With less iodine available, the amount of T1 relative to T2 increases. Less T2 is created relative to T1 since T2 requires two iodides and T1 requires only one.
Increase in thyroidal T3/T4 ratio.
With less iodine available, the amount of T3 relative to T4 increases. For the coupling process, less T2 is available, which makes it harder to create T4 (which requires two molecules of T2). It is easier to create T3 (which requires only one molecule of T2, plus one of T1).
Increase in serum T3/T4 ratio.
With less iodine, serum T3 is much less affected than serum T4. With moderate iodine deficiency, T4 levels may be cut in half, but with severe iodine deficiency, they may become undetectable. Meanwhile, the serum T3 levels remain relatively stable until very severe and prolonged iodine deficiency.
With iodine deficiency, there also appears to be an increase in deiodinases (the enzymes that remove iodide atoms). This increases the conversion of T4 to T3.
Thus, a major adaptation of the body to a severely iodine-deficient diet is a dependence on serum T3 rather than T4.
There is evidence that hypothyroidism can occur in iodine-deficient rats with markedly depressed serum T4 levels, even when T3 levels are normal. (185, 188)