Selenium and thyroid health

Role of selenium

Selenium is a trace mineral which has intermidiate properties between metal and non metal. Selenium exists in two forms: inorganic (selenate and selenite) and organic (selenomethionine and selenocysteine).

Both forms can be good dietary sources of selenium. It can be found in soil and water (inorganic forms), and through the plant roots it enters the food chain (organic form) [1].

Selenium is extremely important for the human body as it improves immunity, acts like an antioxidant and helps with maintaining healthy methabolism. Apart from that, is linked with reduced risk of cancer and autoimmune thyroid diseases [2].

What are the mechanisms of work?

The mechanisms linked to thyroid health include [3]:

• inhibitory effect of HLA-DR molecule expression on thyrocytes
• significant reductions of thyroid stymulating hormone (TSH) receptor antibodies (TSHR-Ab) and TPO antibodies (TPO-AB)
• prevention of dysregulation of cell-mediated immunity and B cell function
• neutralising reactive oxygen species (ROS) and inhibition of redox control processes required for the activation, differentiation and action of lymphocytes, macrophages, neuthrophils and natural killer cells
• inhibition of expression of proinflammatory cytokines
• inhibition of prostaglandin and leukotriene synthesis

Selenium supplementation and disease's process

There is an evidence that starting selenium supplementation earlier in the disease process is associated with greater health benfits. The variable benefit of taking selenium depends on diseases progress and degree of inflammation [4].

The maximal benefit of suppelementation can be achieved in those who are hyperthyroid [3].

Selenium supplementation in treatment of Hashimoto' disease

Gartner et al in 2002 reported significant drop of TPO-Ab in patients who were taking 200mcg daily for 3 months (TPO Ab concentration decreased to 63.6% in selenium group vs. 88% in the placebo group) [4].

In the study from 2008, Toulis et al, proved that 3 months selenium supplementation can significantly lower down TPO-Ab tiers. Another Study from 2012, performed by Bhuyan et al, showed that 200 mcg daily for 3 months, could decrease anti-TPO antibiodies concentration by 49,5% in comparison to 10.1% in the placebo group [7].

Selenium supplementation in treatment of Graves' disease and Ophthalmopathy

In the study from 2011 Marcocci et al proved that Selenium supplementation improved quality of life and significantly slowed the progression of ophthalmopathy (GO) [8].

Another study from 2014, undertaken by Khong et al, showed that serum selenium levels were significantly lower in orbitopathy patients compared with Graves' group. Relative selenium deficiency may be an independent risk for orbitopathy in patients with Graves' diseases [9].

Protective role of selenium on excesive iodine intake

Chronic high iodine intake has been associated in various studies with increased frequency of autoimmune thyroiditis.

In susceptible individuals, iodine excess increases intra-thyroid infiltrating Th17 cells and inhibits T regulatory (TREG) cells development, while it triggers an abnormal expression of tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) in thyrocytes, thus inducing apoptosis and parenchymal destruction [10].

Many Hashimoto's patients have normal iodine levels, but the iodine deficient patient should be extra careful with iodine supplementation to not increase the inflammation.

Selenium supplementation, protects the thyroid cells from oxidative damage from hydrogen peroxide used to incorporate into thyroid hormone [10].

Supplementation not for everyone?

Even if supplementation of selenium is well tolerated, it should not be universally recommended. Checking level of iodine and selenium is recommended instead.

The crucial point is to achieve selenostasis as well as adequate urinary iodine excretion to control disease [10].

As proven above there are no doubts that adequate selenium levels are important to thyroid function. But, we don't know all potential risks of long-term use of selenium supplementation. The safest choice to meet daily requirement is to eat selenium-rich food.

Table 1: Recommended Dietary Allowances (RDAs) for Selenium [11]

Age	Male	Female	Pregnancy	Lactation
Birth to 6 months	15 mcg*	15 mcg*
7–12 months	20 mcg*	20 mcg*
1-3 years	20 mcg	20 mcg
4–8 years	30 mcg	30 mcg
9–13 years	40 mcg	40 mcg
14–18 years	55 mcg	55 mcg	60 mcg	70 mcg
19–50 years	55 mcg	55 mcg	60 mcg	70 mcg
51+ years	55 mcg	55 mcg

*Adequate Intake (AI)

Where we can find selenium?

Although Selenium is present in soil worldwide, factors such as soil composition, plant species and physiological condition of the plant, agricultural practises have a significant influence on Selenium content of veggies, fruits, meat, fish, and water.

Good sources of selenium include: pork, beef, turkey, chicken, fish, shellfish, eggs, beans, nuts particulary brazil one.

Product name	Micrograms (mcg) per serving
Brazil nuts, 1 ounce (6–8 nuts)	544
Tuna, yellowfin, cooked, dry heat, 3 ounces	92
Halibut, cooked, dry heat, 3 ounces	47
Sardines, canned in oil, drained solids with bone, 3 ounces	45
Ham, roasted, 3 ounces	42
Shrimp, canned, 3 ounces	40
Macaroni, enriched, cooked, 1 cup	37
Beef steak, bottom round, roasted, 3 ounces	33
Turkey, boneless, roasted, 3 ounces	31
Chicken, light meat, roasted, 3 ounces	22
Cottage cheese, 1% milkfat, 1 cup	20
Rice, brown, long-grain, cooked, 1 cup	19
Beef, ground, 25% fat, broiled, 3 ounces	18
Egg, hard-boiled, 1 large	15

The risks from excecssive selenium

Chronic high intake of selenium leads to serious health problems. Early indcitators are: garlic breath and metallic taste in the mouth.

The most common clinical signs of selenosis are hair and nail loss or brittleness.

Other symptoms which may occur are: lesions of the skin and nervous system, nausea, diarrhoea, skin rashes, fatigue, irrability, and nervous system abnormalities.

As mentioned before, brasil nuts are great source of selenium, but consumed regularly can lead to selenium toxicity. Moreover there is a risk of consuming aflatoxin produced by Aspergillus flavus (nuts are susceptible to develop this mould on their surface) which may be present on the nut.

Regular consumption in this case will lead to chronic toxicity of aflatoxin, which further can potentially increase the risk of liver cancer.

Knowledge is power

Armed with the above information, we could start more consciously discovering what will be the best option to support thyroid gland.

Knowing the risks and benefits can help us to make better choice and improve our health.

Performing blood test of iodine and selenium, increasing consumption of selenium rich food or when necessary starting selenium supplementation will help to improve thyroid health.