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A link between intelligence and goiter had been known since antiquity.

But it wasn’t until the discovery of iodine inside the thyroid gland in 1896 that science started to understand goiter.

In an iodine-deficient state, the thyroid swells-up in an attempt to filter more blood — it needs iodine from the blood.

This swollen thyroid is a goiter.  

The goiter impairs thyroid function, ultimately causing hypothyroidism.

It’s not the only thing that causes hypothyroidism, but it’s the one we’re looking at today.

And today, we’re focused on the link between low iodine and low intelligence.

In fact, there is a negative association between sub-clinical iodine deficiency and IQ.

Dutch researchers chose 21 of the best studies from four continents for their meta-analysis.
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Every single study analyzed showed a positive correlation between IQ and iodine.

This is because the thyroid needs iodine to create thyroxine (T₄), a hormone with four iodine atoms.

Without this thyroid hormone, you see impaired neurological growth.

And metabolism slows.

After all of the data was considered, they found that the difference between the two groups was 13.5 IQ points.

Concretely, this means that the mean scores for the two groups studied, the iodine-deficient group and the noniodine deficient group, are 0.9 SD, or 13.5 IQ points, apart.

Similar reductions in IQ can happen with other restrictions to thyroxine (T₄) synthesis, such as the iodine antagonists perchlorate and thiocyanate.

These bind strongly to the thyroid and displace iodide.

Perchlorate binds the strongest but is relatively rare; cruciferous vegetables, radishes, and mustard seed contain thiocyanate.

Low iodine is just one potential cause of hypothyroidism, but an important one.

Eleven years later, Chinese researchers did another meta-analysis.

They chose the best 37 studies in China and found a 12.5 IQ point difference between iodine deficient (ID) and iodine sufficient regions.

The intelligence of children who lived in ID areas with no iodine supplementation had an average of 12.45 IQ points lower than that of children who lived in a naturally IS environment.

They corrected for socioeconomic factors and noticed an increase in IQ following widespread salt iodination.

But many people have stopped eating iodized salt.

It often has sodium aluminosilicate added as an anti-caking agent.

Many people now eat non-iodized sea salt.

Also, iodine was reduced as a dough conditioner the late 70s and early 80s.

So people are generally getting less iodine than they did 50 years ago.

A 1998 study of iodine nutrition in the United States using data from NHANES showed a decrease in median urine iodine concentration from 320 μg/L in 1971–1974 to 145 μg/L in 1988–1994.

About 11.1% of the US population has iodide levels that would be considered low in many studies.

Even though hypothyroidism is more dangerous for developing children, it does affect the IQ of adults as well.

They even have a name for it: hypothyroid dementia.

The medical authors describe two case reports of a reversal of dementia with thyroxine (T₄).

The first case responded after a few months with 75 μg thyroxine; the second one needed 175 μg thyroxine.

Great increases in IQ scores were noted.

So why is this? Why does reduced T₄ cause dementia?

The first thing that comes to mind is that there is a lower rate of metabolism in the hypothyroid brain and fewer thoughts per minute.

There may be some truth to this, but researchers discovered something else as well:

Friends and family of a subject encouraged her to go the hospital because they noticed she was becoming progressively more forgetful, lethargic, and confused.

They reported mildly depressed mood and mildly diminished verbal fluency and activity at that time. A month later, work colleagues noted apathy, inability to recall schedules and duties, and decreased productivity.

She was diagnosed with hypothyroidism based on high TSH, injected with chelated radioactive technetium, and imaged.

This technique measures blood flow, and is a common way to measure brain perfusion.

They noted reduced blood flow in the hypothyroid subject.  

And 75 micrograms of thyroxine improved her condition immensely.

Supplemental thyroid hormone may be necessary in some cases, but it’s probably better to have the thyroid make its own.

Researchers don’t know what iodine does for the body in other wyas yet.  But they do know that it’s necessary for blood and thyroid health.

Scientists consider iodine a micro nutrient because they measure it in micrograms (μg).

But how much iodine do you need?

The RDA is 150 μg/d, with 1,100 μg/d as the recommended upper limit.

This seems reasonable, although I would feel more comfortable towards the middle part of this range (~500 μg/d).

The average intake in Japan is 1,200 μg/d due to their high seaweed consumption (Nagataki 2008).

The dairy industry also uses iodine and milk usually contains about 400 μg per liter (Pearson 2004).

But taking iodine for thyroid health is a good plan to lower your risk of dementia.

Like most vitamins, iodine follows a U-shaped curve. 

We know that too little is bad for your thyroid.  

But too much iodine can also cause thyroid issues.

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Matt Cook is editor-in-chief of Daily Medical Discoveries. Matt has been a full time health researcher for 26 years. ABC News interviewed Matt on sexual health issues not long ago. Matt is widely quoted on over 1,000,000 websites. He has over 300,000 daily newsletter readers. Daily Medical Discoveries finds hidden, buried or ignored medical studies through the lens of 100 years of proven science. Matt heads up the editorial team of scientists and health researchers. Each discovery is based upon primary studies from peer reviewed science sources following the Daily Medical Discoveries 7 Step Process to ensure accuracy.