Iodine and Thyroid Health
The main function of the thyroid gland is to make the thyroid hormones T4 and T3.
Just like building anything else, effective production depends on the availability of raw material, efficient machinery and appropriate controls.
When it comes to chronic disease, many of our problems begin with nutrient deficiencies.
Iodine is a critical raw material for thyroid hormone production.
65% of T4 is iodine.
The thyroid takes iodine and converts it into thyroid hormones: thyroxine (T4) and triiodothyronine (T3).
Thyroid cells are the only cells in the body which can absorb iodine.
These cells combine iodine and the amino acid tyrosine to make T3 and T4.
Most of what is produced is T4 and T4 must be converted to T3, primarily by two enzymes known as D1 and D2 and trace amounts of selenium.
T3 and T4 are then released into the blood stream and transported throughout the body where they control metabolism (conversion of oxygen and calories to energy).
This is why iodine and thyroid health (and selenium) are intimately connected.
When you're deficient in iodine, thyroid hormone production will be lowered.
But the body can compensate through an increase in thyroid stimulating hormone (TSH) and increased D2 activity, which then results in increases in T4 to T3 conversion.
Here's where it gets tricky.
If you have both a selenium deficiency and iodine deficiency, it will reduce D2 activity and impair this compensation mechanism.
This is why we cannot simply focus on a single nutrient, but need to recognize that all nutrients are essential for optimal function!
Iodine and Hypothyroidism
Because Iodine is essential for the production of thyroid hormone, it shouldn't be a shocker to find out that iodine deficiency is the most common cause of hypothyroidism worldwide.
This is one of the primary reasons why we have iodized salt.
And while this strategy was initially effective, it also had an unexpected and undesired consequence.
In countries where iodine has been added to table salt, the rates of autoimmune thyroid disease have risen? [1, 2, 3, 4, 5]
Why does that happen? We don't really know!
But we do know that the association between iodine intake and the presence of thyroid antibodies occurs at both high and low levels of iodine. 
One study even found that 78% of patient's with Hashimoto's regained normal thyroid function with iodine restriction alone. 
All that said, Iodine is obviously important to prevent hypothyroidism, but too much iodine can lead to autoimmune hypothyroidism.
How to Test Iodine Deficiency
There are certainly lab tests that can be done to assess iodine status and deficiency, but let's not forget good old physical examination signs and symptoms.
One of the first signs of iodine deficiency is decreased sweating as the body tries to decrease iodine clearance. And once that happens, clear signs of iodine deficiency emerge.
Some of the most common signs and symptoms mirror that of hypothyroidism such as :
- Brittle fingernails
- Hair loss, diffuse
- Puffy eyes
- Subnormal body temp
- Decreased sweating
- Hoarse speech
- Skin dry/coarse
- Thyroid enlargement
- Edema (non-pitting)
- Lateral 1/3 eyebrow loss
- Slow heartrate
- Hair dry/coarse
- Slow reflex return
In my patients, I use a combination of three tests:
- 24-hour urine iodine: to assess recent iodine intake
- Serum thyroglobulin: high thyroglobulin indicates low iodine status, and levels above 40 mcg/L are suggestive of deficiency 
- Hair iodine: to assess long-term iodine intake 
How to Correct Iodine Deficiency
If you have an iodine deficiency, then you will want to work at increasing both iodine and selenium.
Selenium helps to protect against iodine toxicity. 
Selenium is also required for the proper conversion of T4 to T3.
To increase selenium, consume brazil nuts, cod, shrimp, tuna, halibut, salmon, scallops, eggs and shitaki mushrooms.
To increase iodine, increase kelp, other forms of seaweed and of course iodized salt.
If supplementation is necessary, start with a very low dose, perhaps 100 micrograms of iodine.
If necessary, and under the guidance of healthcare supervision, 325 mcg of kelp tablets per day should suffice.