Whenever necessary, T4 in the blood converts to a more active T3 hormone by releasing one iodine atom. Maintaining body temperature during cold is such an example: On exposure to cold, T4 in the bloodstream converts to T3 and the thyroid receptors around lever, heart, kidneys and other vital organs initiate the process of burning fat to raise the body temperature.
Despite the small size, thyroid gland produces some of the most important hormones necessary for a baby during pregnancy. Therefore, adequate supply of iodine is extremely important for the development of infants. Research has shown a strong correlation between T4 levels of mothers and the IQ of babies after they grow into adults. Insufficient iodine during pregnancy therefore can have long-term impact on society.
Iodine is commonly found in seafoods. Therefore, coastal regions tend to have excess supply of iodine. On the other hand, regions further inland, with limited access to seafoods, tend to be iodine deficient. To meet the daily iodine requirement, salt is often fortified with iodine and sold as iodized salt.
This approach has been very successful in reducing mental disorders among newborns in some regions of developing countries that lacked iodine in their diets. It has also helped reduce cases of goiter—which appears as a swollen neck when the thyroid gland enlarges in order to produce adequate levels of T3 and T4. Insufficient iodine is one of the most common causes of goiter besides thyroid dysfunction.
Although excess iodine eventually leaves the body via urine, over long periods it can result in high levels of thyroid hormones causing hyperthyroidism. Commonly observed symptoms include hyperactivity, with a faster metabolism and constant feeling of being hungry. A more visible sign is the intolerance to heat, accompanied with other symptoms including anxiety, nervousness, sweating, weakness of muscles, and heart palpitations.
Adequate supply of iodine is important for maintaining a tightly controlled range of thyroid hormones. Whenever T3 and T4 levels aren’t optimal, e.g., during insufficient iodine supply, very opposite symptoms are observed and the condition is termed hypothyroidism. Typical signs include high sensitivity to cold, weight gain and slow response.
Another related key hormone released by the pituitary gland in our brain is called TSH (thyroid stimulating hormone). TSH is the main switch that balances adequate thyroid hormone levels. In case of thyroid dysfunction, TSH levels will drive the gland to compensate for excess or low T3 and T4—up until a certain level. Once this subclinical threshold is reached, an overt hypothyroidism or overt hyperthyroidism might require immediate medical attention. Beside inadequate supply of iodine, age and pregnancy can also throw off this delicate balance maintained by TSH.
In rare cases, the body’s immune system starts to attack thyroid gland—often due to an auto-immune disease. It follows production of antibodies called TPO (thyroid peroxidase), which can be detected with a comprehensive thyroid blood test. The test checks for free-T3, free-T4, TSH and TPO antibodies. Regular testing is an excellent way to ensure people consume healthy amount of iodine in their diet.