Iodine/Iodide

Iodine (oxidized) is a halogen (nonmetallic element containing seven valence electrons) and is often found negatively charged because it gains an electron to make an octet; due to this, iodine is often interchangeably called by its anion name, iodide (reduced).¹ This text will refer to it as iodide unless specifically noted as iodine.  Iodide is found within the human body ranges between 15-20 milligrams, and deficiency of iodide is known to cause goiter (enlargement of the thyroid gland).¹ A commonly known source of iodine is salt - the iodination of salt in the U.S. occurred due to the high occurrence of goiter. 

Absorption and Excretion: Iodide from biliary secretions (dependent on iodide status) and diet is absorbed along the GI tract, including the stomach.¹ Nearly all iodide is absorbed within the body and excessive iodide is excreted through urination.¹ Iodide in the blood dissolves and fecal iodide excretion is minimal.¹

Storage: 75-80% of iodide in the body is found within the thyroid gland. The remaining iodide is found within salivary and gastric glands.¹ Within the thyroid and salivary gland, iodide uses an aggressive active transport system to bring iodide within cells from the extracellular fluid. Iodide is required to produce thyroid hormone, and 60 micrograms of iodide is required daily to produce adequate thyroid hormone.¹

Metabolism and Circulation: The primary function of iodide is to create thyroid hormone. To begin the process, iodide is oxidized to iodine within the thyroid cells. It is incorporated into a globulin protein called thyroglobulin - a storage form of iodine.¹ To be removed from storage, the thyroglobulin protein is iodized when the amino acid tyrosine reacts with iodine in certain positions along the molecule to form two types of thyroid hormones: thyroxin (T4) and triiodothyronine (T3).¹

90% of T4 is released from the thyroid gland daily compared to 10% of T3; however, T3 is 5-10% more potent than T4.¹ Thyroid hormone binds to thyroxine-binding protein (80%), thyroxine-binding prealbumin (10-15%), and albumin (5-10%) when circulating in the blood.¹

The amount of thyroid hormone released into the blood is controlled by the hypothalamus of the brain. It releases thyrotropin-releasing hormone (TRH) that stimulates the pituitary gland to release thyroid-stimulating hormone (TSH).¹ TSH targets thyroid cells to produce thyroid hormones T3 and T4 which enter the blood and peripheral tissue. The production of thyroid hormones provides negative feedback (stopping overproduction and promoting a regulated routine release) of TRH and TSH (to a certain extent).¹ T4 is converted to T3 within peripheral tissues.¹

Function: The function of iodide creates thyroid hormone which has multiple metabolic functions. In order for thyroid hormones to exert their effects, T4 binds to the thyroid receptor along the DNA promoter region.¹ The thyroid receptor with T4 can form a dimer with itself or with retinoid (derived from vitamin A) X receptors, or RXR.¹ The thyroid receptor and/or RXR dimer complex begins transcription at the gene level when T3 binds to the thyroid receptor.¹ At this point, the complex can suppress or induce certain genes.¹ Examples of thyroid hormone effects include:

Metabolism: 

Carbohydrates - Thyroid hormone promotes glucose absorption, enhances glycolysis and gluconeogenesis, and promotes insulin release.¹

Fat - Thyroid hormone mobilizes fat from adipocytes, increases fatty acid presence and oxidation in cells and decreases plasma levels of cholesterol and triglycerides.¹

Protein - Thyroid hormone promotes protein synthesis if not present in excessive amounts (can induce protein catabolism).¹

Metabolism - Thyroid hormone increases the general metabolism of cells; lack of it will lessen basal metabolism.¹

Systematic Responses:

Respiratory - Respiration occurs more due to increased cellular metabolism (induced by thyroid hormone).¹

Cardiovascular - Increased stroke force and heart rate with little change in blood pressure (potentially elevated systolic pressure and decreased diastolic pressure).¹

GI Tract - Stimulates appetite, promotes gastric motility, and increases gastric juice secretion.¹ A lack of thyroid hormone can cause constipation. 

Central Nervous System - The feeling of excitability is increased; too much thyroid hormone may produce nervousness.¹

Endocrine - Secretions from the majority of endocrine glands release faster with the production of thyroid hormone.¹

Muscle - The force of muscle contractions is increased due to the production of thyroid hormone.¹

Dietary Reference Intake (DRI) - The Recommended Daily Allowance (RDA) of iodide is 150 micrograms for men and women over 14 years old.² Pregnancy and lactation increase the need for iodide 220 micrograms and 290 micrograms.²

Deficiency - A deficiency of iodide leads to less thyroid hormone production in the thyroid gland.¹ Iodide deficiency in childhood can cause mental deficits, poor growth, inadequate organ maturation, and delayed sexual maturity.¹ Iodide deficiency results in goiter, an enlargement of the thyroid gland. If infants are born to mothers with low dietary iodide consumption, cretinism (mental and physical deficits - presents with large head, deaf, and coarse features) can occur.¹ A higher chance of pre-eclampsia, low birth weight, miscarriages, and stillbirths can occur due to iodide deficiency.¹ While not common in the U.S., many adults and children around the world (Southeast Asia, East and South Africa, Europe, Central America, Himalayas) develop iodide deficiency and can have disorders associated with it.¹ ²

Toxicity - Excessive iodide levels can present as iodide deficiency - this is because excessive iodide signals thyroid hormone production to stop.¹ TSH is stimulated to produce more thyroid hormone and can also cause goiter.² The tolerable upper limit for males and females 19 years old and older is 1100 micrograms.²

Dietary Sources of Iodide

Iodide is known for being a component in iodized salt - this was due to the prevalence of iodide deficiency in the U.S. Iodide content in water is derived from minerals and rocks from which the water passes through.¹ Iodide content from plants is dependent on the soil in which the plants are grown.¹ In animals, iodide content is dependent on the plants they eat and the water they drink.¹

Fish and seafood are good sources of iodide compared to fruits and vegetables. Meat, eggs, and dairy products also contain iodide. Seaweed contains a high amount of iodide. Breads can contain iodine through the use of dough conditioners and may not be listed in the Nutrition Facts panel.²

Examples of foods containing iodide include

High Iodide Containing Foods - Iodized salt, seaweed

Grains - Bread with iodate conditioners

Fish/Seafood - cod, oysters, shrimp, tuna 

Dairy/Animal Products - milk, yogurt, egg 

Source(s):

1. Denise M Medeiros and Robert E.C. Wildman, Advanced Human Nutrition, 4th ed. (Burlington, MA: Jones & Bartlett, 2019).

2. https://ods.od.nih.gov/factsheets/Iodine-HealthProfessional/