Thyroid function is important for the metabolism of almost all tissues and the development of the central nervous system in the fetus and newborn. The effect on the thyroid is caused by two iodine-containing hormones, triiodothyronine (T3) and thyroxine (T4). Iodine (atomic number 53, standard atomic mass 126.9) is an element that limits the rate of synthesis of thyroid hormones. Currently, the only known physiological role of iodine in the human body is the synthesis of thyroid hormones by the thyroid gland.

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The link between iodine deficiency and thyroid disease has been known since the beginning of the 20th century. Iodine deficiency is considered one of the leading causes of preventable brain damage worldwide.  Although the prevalence of severe iodine deficiency has decreased recently, the problem of iodine deficiency is re-emerging in vulnerable groups such as pregnant women and infants. Also, some foods and medicines are very high in iodine, which can cause thyroid dysfunction in sensitive people.

Iodine is a trace element in soil and water that is absorbed in a variety of chemical forms. Many forms of iodine are reduced to iodide in the gut. Iodine is almost completely absorbed in the stomach and duodenum. Iodine is mainly excreted from the blood through the thyroid gland and kidneys.  

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Under normal conditions, the plasma has a half-life of iodine which is about 10 hours, but the half-life is shortened when the thyroid gland is overreacting, such as iodine deficiency or hyperthyroidism. The average daily circulation of iodine in the thyroid gland is about 60-95 mcg in adults in areas where iodine is sufficient. A healthy adult has 15-20 mg of iodine in the body, of which 70-80% is in the thyroid gland.

When T4 and T3  are degraded in the periphery, iodine is released and returned to the plasma iodine pool.

Most of the ingested iodine is eventually excreted in the urine. Only small amounts appear in the stool. The mammary glands take care of the newborn by concentrating iodine and distributing it in breast milk. Salivary glands, gastric mucosa and choroid plexus also absorb small amounts of iodine. 

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NIS and pendrin have been recorded in the trophoblast, and the amount of iodine in the placenta is about 3% of that of the thyroid gland.

Iodine is known to regulate thyroid function.  The main effect is to reduce the thyroid’s response to thyroid-stimulating hormone (TSH). Strong inhibition of auto-oxidation; reduce catch after delay. It inhibits the secretion of thyroid hormones at high concentrations. A small change in iodine intake is sufficient to alter serum TSH levels in the thyroid system.   

This suggests that iodide-induced regulation of the thyroid response to TSH plays an important role in the negative feedback loop. In response to increasing the iodine dose, tissue iodine initially increases and then decreases. This rapid inhibition of these tissues, called the”wolf-chaikoff effect”, occurs because of the high concentration of inorganic iodide in thyroid cells. The mechanism by which the body suppresses it is unclear, but may be related to the inhibitory effect of iodide on thyroid peroxidase or other enzymes.

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Iodide has been reported to inhibit several metabolic steps in thyroid cells. Iodine also activates the production of H2O2, which activates protein iodination in the thyroid gland of some species, including humans.  

Here are a few sources which are rich in Iodine: 

• Cod 
• Plain low-fat yogurt
• Reduced fat milk 
• White enriched bread 
• Shrimp 
• Egg 
• Canned tuna in oil 
• Dried prunes 
• Cheddar cheese 
• Raisin bran cereal
• Apple juice 
• Frozen green peas 
• Banana 

As iodine is needed to make thyroid hormones, low iodine levels can cause hypothyroidism (low thyroid function). Iodine deficiency is also associated with the development of goitre (an enlarged thyroid gland). The effects of too little iodine are exacerbated.  

Children born to mothers with severe iodine deficiency may suffer from pyrosis, severe and irreversible mental retardation, and motor, speech, and hearing impairments. Although many children improve with iodine supplementation, even mild iodine deficiency during pregnancy can lead to intellectual disability.

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Mild iodine deficiency can also cause miscarriage. Fibrocystic breast disease is usually a benign, painful tumour in women of childbearing age and is associated with iodine deficiency.

Some healthcare providers believe that people with thyroid problems need iodine supplements. Alternatives health care providers may recommend herbs that contain iodine such as kelp or seaweed. This can be especially dangerous, in part because iodine supplements can interact with many types of medications, including anti-thyroid drugs used to treat hyperthyroidism. If iodine deficiency is not the cause of hypothyroidism, iodine supplements will not help.

You should be very careful about increasing your iodine intake unless you and your healthcare provider have very strong evidence that you are deficient. This is especially true if you are pregnant or planning to become pregnant.

In some people with thyroid disease, too much iodine can cause or worsen hypothyroidism.  You may have more energy at first, but high doses can cause "iodine depletion” that can make you feel tired and sick within days. 

High iodine intake can initiate and increase thyroid infiltration by lymphocytes, the white blood cells that accumulate as a result of chronic injury or irritation.

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Too much iodine also inhibits the ability of the thyroid gland to produce thyroid hormones. A 2014 study in the journal of  Endocrinology and Metabolism found that too much or too little iodine is dangerous and can lead to hypothyroidism and autoimmune thyroiditis (Hashimoto's thyroiditis, chronic lymphocytic thyroiditis), especially in people with recurrent thyroid disease.

According to a 2012 study published in the Journal of paediatrics, women who take too many iodine supplements during pregnancy may have children with congenital hypothyroidism. Iodine poisoning is rare, but consuming too much iodine can be just as problematic as consuming too little.

Hence, both iodine deficiency and iodine excess are associated with an increased risk of thyroid disease. Also, further research is needed to determine the optimal range of iodine intake and to clarify the effect of iodine intake on thyroid disease.

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