Iron is an element essential for many vital functions in the body, such as formation of hemoglobin, and thus red blood cells that are required for almost all cells and processes to work properly.
There are 2 forms of iron, including heme and non-heme iron.
The relationship between iron and thyroid is interchangeable, which means that both of them can affect the other.
Generally, the lower thyroid hormones are produced, the lower iron is absorbed, and vice versa.
How can iron levels affect thyroid gland functions?
- Preliminarily, iron deficiency causes anemia, whose symptoms are kind of similar to those of hypothyroid, such as easily feeling fatigue, a general complaint of weakness, achiness, palpitations, hair loss, a noticeable fast heartrate, and being likely to losing sex drive.
- In order to form both hormones of the thyroid, including thyroxine (T4) and triiodothyronine (T3), an enzyme named thyroid peroxidase needs to bind to a certain component of hemoglobin, which is known as heme I. Iron deficiency can result in decreasing the levels of thyroid hormones (hypothyroidism), as a consequence of iron effects on hemoglobin formation.
- In an experiment conducted in rats, iron deficiency decreased both the active form of thyroid hormones (triiodothyronine (T3)), as well as deiodinase enzyme. 
- Iron deficiency also inhibits the first 2 steps of thyroid-hormones synthesis through lowering the activities of thyroid peroxidase enzyme, as mentioned above. 
- According to a study of 431 adolescent girls, 103 of them are iron deficient underwent different tests that assessed “the levels of thyroxine (T4), triiodothyronine (T3), free thyroid hormones (fT4 and fT3), iodine, ferritin, iron, total iron binding capacity (TIBC), thyroid stimulating hormone (TSH), triiodothyronine resin uptake (T3RU), reverse triiodothyronine (rT3), selenium and albumin concentrations.” It was concluded that iron deficiency is likely to affect the status of thyroid hormones in the body. 
How can the thyroid affect iron levels?
Through different mechanisms, thyroid is likely to affect iron levels in the blood, as follows:
- Firstly, in contrast with the second point mentioned above, hypothyroidism can also negatively affect a certain process in the liver known as heme oxidation. That in turn results in iron deficiency.
- Low levels of thyroid hormones (hypothyroidism) decreases the production of gastric juice, which in turn results in decreasing the absorption rate of iron and other nutrients from the stomach. According to a study conducted in 2000, the levels of 3 substances, including pentagastrin stimulated-acid, histamine and carbachol, were lowered in hypothyroid rats; while that was reversed in hyperthyroid rats. That led to concluding that thyroid hormones likely exert their effects on the stomach through changing the size or number of the cells responsible for gastric-acid secretion. 
- According to Antonijević N., et al. (1999), pernicious anemia is much likely to occur in patients with hypothyroidism, equating to 20 times more than in those with normal thyroid. Additionally, macrocytic anemia was found in 55% of patients with hypothyroidism. 
How much iron should people take?
Iron should be taken on daily basis, and cautiously. The Food and Nutrition Board established 2 types of values in order to help people avoid the harmful effects and disorders that are likely to occur in cases of both iron deficiency and excessive iron intake .
Recommended Dietary Allowances of iron
- Both male and female infants aged 0-6 months are recommended to take 0.27mg iron/day; while those ranging from 7 to 12 months are recommended to take more iron up to 11mg/day.
- The Children ranging from 1 to 3 years are recommended to take 7mg iron/day; while those aged 4 to 8 years should take 10mg iron daily.
- The children aged 9-13 years should get 8 mg iron daily, and so should the adults ranging who are 51 years or older.
- The male children ranging from 14 to 18 years are recommended to take 11mg of iron daily; while the females are recommended to take up to 15 mg of iron per day.
- The male adults ranging from 19-50 years are recommended to take 8 mg of iron/day; while females are recommended to take up to 18 mg daily.
- The pregnant women aged 14-50 years should consume 14mg of iron daily; while that amount in breast feeding women, as follows: The breast-feeding women who range from 14 to 18 years should only consume 10mg of iron daily; and not more than 9 mg daily for those aged 19-50 years.
Tolerable Upper Intake Levels of iron
The Tolerable Upper Intake Levels show the maximum amount of calcium that can be safely administered daily, as follows :
- Since birth and up to 13 years, the daily intake of iron shouldn’t exceed 40 mg.
- Both pregnant and breast-feeding women who range from 14 to 50 years are allowed to consume up to 45mg of iron daily.
The best sources of iron
In order to get sufficient amounts of iron, 2 point basically should be taken into consideration, including:
- Certain substances and foods can enhance the absorption of iron as well as its bioavailability in blood, such as ascorbic acid, meat, poultry and seafood.
- On the other side, other substances are likely to inhibit iron absorption and its bioavailability in blood, including calcium, phytates and polyphenols. Grains and beans are examples of phytates-containing foods; while cereals, legume and spinach are examples of the foods containing polyphenols.
Therefore, it is advised to consume a diet composed of much variant foods, so that you can maintain a balanced level of iron, overcoming the effect of both inhibitory and stimulatory substances.
According to the National Institute of Health, the foods listed below should be considered so as to get sufficient amounts of iron .
- Lean meat and seafood are the highest foods containing heme iron.
- On the other side, nuts, beans, vegetables, and fortified grain products are the richest sources of non-heme iron.
- bread, cereal
- In terms of babies less than 2 months, breast milk is a good source of iron.
- The majority of infant formulas contain around 12mg of iron.
Along with foods, baby formulas, it’d be beneficial to consider iron-containing-dietary supplements so as ensure getting adequate amounts daily, especially for people who are much likely to iron deficiency, including the 9 groups shown below:
- People with cancer
- Frequent blood donors
- People with heart failure
- Infants and young children
- Women with heavy menstrual bleeding
- People who underwent a gastrointestinal surgery
- Pregnant women
During pregnancy, there is an increasing demand of blood that passes placenta to the fetus. Therefore, more iron is required to form hemoglobin. Low iron levels make pregnant women prone to die, in addition to making the fetus prone to both premature birth and low birthweight.
- People who don’t eat variant foods, especially the elderly and the busy people who are much likely to not pay attention to what they eat/or how much iron they get from each meal.
- People who are struggling with issues that can negatively affect the absorption rate of iron from the gut, such as irritable bowel syndrome, Crohn’s disease and ulcerative colitis.
 Beard J., et al. Evidence for thyroid hormone deficiency in iron-deficient anemic rats. J Nutr. 1989 May;119(5):772-8.
 Hess SY., et al. Iron deficiency anemia reduces thyroid peroxidase activity in rats. J Nutr. 2002 Jul;132(7):1951-5.
 Eftekhari MH., et al. The relationship between iron status and thyroid hormone concentration in iron-deficient adolescent Iranian girls. Asia Pac J Clin Nutr. 2006;15(1):50-5.
 Rafsanjani FN., et al. Effects of thyroid hormones on basal and stimulated gastric acid secretion due to histamine, carbachol and pentagastrin in rats. Saudi Med J. 2003 Apr;24(4):341-6.
Antonijević ., et al. [Anemia in hypothyroidism]. Med Pregl. 1999 Mar-May;52(3-5):136-40.
 Food and Nutrition Board. DRI Tables and Application Reports. United States Department of Agriculture. National Agricultural Library
 Office of Dietary Supplements. Iron. Fact Sheet for Health Professionals. National Institute of Health. Updated: February 11, 2016.