Iodine's Role in Thyroid Function
Iodine is Essential for the Production of Thyroid Hormone
Mar 14, 2009
Iodine is an essential mineral, found mainly in the oceans, necessary for thyroid function. It forms part of thyroid hormones and plays a role in development, growth, and metabolism. Humans cannot function without iodine; it is an essential mineral vital for life and must be a regular part of the diet.
A non-metallic mineral element, iodine occurs mainly as the inorganic form, iodide. This form is more stable and well used by tissues such as for thyroid function. However, iodine foods also provide organic iodine. For example, the iodine rich food, seaweed, contains both inorganic and organic iodine. Therefore, the body requires mechanisms to absorb both forms and to convert the organic iodine to the inorganic iodide, when required.
Iodine Absorption—the Mechanisms
In the small intestine, facilitated diffusion enables organic iodine absorption to occur across the intestinal cells. The more common inorganic iodide form requires a specific transport protein called the sodium-iodide symporter (NIS).
Once iodine absorption is complete and the iodine is in the blood stream both forms are concentrated into various tissues including the thyroid, mammary glands, the eyes, salivary glands, cervix, thymus, choroid plexus, kidneys, arteries, and bones. These tissues contain the sodium-iodide symporter, so they can absorb the inorganic form.
The Main Iodine Function
The thyroid is the main user of iodine for thyroid function. Using the sodium-iodide symporter, iodine is absorbed within the thyroid gland follicle at the rate of approximately 60 micrograms (mcg) per day; however, optimal thyroid function needs a daily intake of 150mcgs per day.
Iodine is essential for the production of thyroid hormone. The two thyroid hormones are thyroxine (T4) and triiodothyronine (T3). These hormones are dependent on iodine and the amino acid tyrosine.
Thyroid hormone regulates:
- Growth
- Development (especially fetal development)
- Metabolism
- Reproductive function
The role of iodine beyond thyroid function is largely unknown; however, iodine mammary gland concentration is important for fetal and neonatal development.
Thyroid Hormone Production
The number of iodine molecules each thyroid hormone contains is the reason for the abbreviations – T4 and T3. Iodine makes up 64 percent of the molecular weight of T4 and 59 percent of T3.
The process of thyroid hormone production begins with the brain recognising a low blood level of thyroid hormones.
In the brain:
Low thyroid hormone levels detected. The hypothalamus releases thyrotropin-releasing hormone (TRH). TRH stimulates the pituitary gland to release thyroid-stimulating hormone (TSH).
In the thyroid:
The TSH production stimulates the sodium-iodide symporter to transport iodine from the blood into the thyroid follicle. This increases the levels within the follicle to 20-40 times greater than the blood, and traps the iodine within the thyroid.
Any organic iodine is oxidised to the inorganic iodide by an enzyme – thyroid peroxidase(TPO).
In the thyroid follicle a molecule called thyroglobulin (TGB) is produced which contains tyrosine. Iodine, only as the inorganic form iodide, attaches to the tyrosine to produce T3 and T4.
Once produced, enzymes break the T3 and T4 away from the TGB molecule and they enter the blood stream.
Back to the brain:
When the pituitary detects an increased level of thyroid hormones in the blood, it reduces the production of TSH, which in turn, reduces the production of these hormones. This is negative feedback.
A large proportion of the thyroid hormone released is T4; however, it converts into the more active T3 within tissues, especially the liver. This process requires the essential, and often deficient, mineral selenium. Therefore, for adequate thyroid hormone production and use, the body also needs enough selenium and a healthy liver.
An Essential Mineral
Iodine is essential for thyroid hormone production and thyroid function. A regular supply of iodine foods is crucial. Levels of this essential mineral have been declining worldwide over the past 10-15 years. The impact of iodine deficiency affects individuals and communities; iodine deficiency affects the whole population.
References:
Patrick, L. (2008). Iodine: deficiency and therapeutic considerations. Altern Med Rev, 13(2), 116-127.
Rheault, S., Olmstead, S., Ralston, J., & Meiss, D. (2008). Iodine and Iodide: Functions and Benefits Beyond the Thyroid. Townsend Letter for Doctors & Patients, pgs 99-101.
Tortora, G. J., & Grabowski, S. R. (1993). Principles of Anatomy and Physiology. 7th ed (7th ed.). New York: Harper Collins.
Guyton, A. C., & Hall, J. E. (2006). Textbook of medical physiology (11th ed.). Philadelphia: W.B. Saunders.
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