Abstract: 

Pregnancy is a state that demands increased functioning of the ever-important tiny thyroid gland. The thyroid gland increases in size and produces more hormones during pregnancy. A properly functioning thyroid leads to a smoothly run pregnancy, minimizing miscarriage risk, and supporting proper fetal brain development. Selenium is one of the essential nutrients for thyroid functioning and hormone production. Selenium can be found in several foods or a dietary supplement, to at least reach the RDA of 60 mcg/day. To avoid any selenium toxicity effects, mothers should also be cautious of the 400 mcg/day tolerable upper limit.   

What Is Selenium? What Role Does It Play In The Body? 

Selenium is an essential micronutrient that H4: actively promotes thyroid function. The thyroid gland has a high concentration of selenium, which is incorporated into more than two dozen selenoproteins. These proteins play a central role in reproduction, thyroid hormone metabolism, and antioxidant activity directed against free radicals created from thyroid hormone production.

Selenium is necessary for the maintenance of other cellular functions, as well, including immune-endocrine function, signaling transduction pathways, metabolism, cellular homeostasis, DNA synthesis, and protection from oxidative damage and infection. 

Let's Start With Thyroid Health and Why It's Critical To Pregnancy…

The thyroid is a small butterfly-shaped gland that sits low on the front of your neck. This gland secretes important hormones, T3, T4, and TSH, which significantly impact all aspects of your metabolism and everyday health.  

Our body is very interconnected, specifically our metabolic and reproductive systems. When the thyroid is operating properly, H4: the necessary levels of thyroid hormones are produced, which assist with regular menstrual cycles and optimal fertility. Overall, thyroid health has a sizable impact on fertility and pregnancy success. Proper treatment of thyroid disorders helps mothers have a healthy pregnancy by carrying a baby to full term, optimizing birth weight, and supporting the healthy development of the placenta and the baby's brain.

One of the main thyroid hormones, T4, requires many nutrients for production, including selenium. Even with normally functioning thyroid glands, T4 production can get disrupted quite easily. It is dependent on adequate amounts of a long list of nutrients and is heavily influenced by endocrine disruptors, such as stress, infection, physical trauma, radiation, and certain medications.

Furthermore, pregnancy is a state of excessive thyroid stimulation, leading to an increase in thyroid size by 10%-40%. Pregnancy demands more from our thyroid, increasing thyroid hormone production immensely, as maternal estrogen levels stimulate increased thyroxine (T4) binding globulin production. In the early stages of pregnancy, predominantly the first trimester, the baby is entirely dependent on the mother's thyroid hormone production. The baby's thyroid hasn't developed yet! Thyroid hormone is needed in increased amounts right away but needs peak between weeks 6-10 of pregnancy when miscarriage risks are still relatively high. One of the leading causes of early pregnancy loss is thyroid dysfunction. However, a proper functioning thyroid is still critical throughout the pregnancy, as maternal thyroid hormones continue to impact normal growth and development of the baby. 

Evidence has accumulated over the years indicating that T4 plays a significant role in the normal development of the fetal brain. Specific nuclear receptors and thyroid hormone function found in the fetal brain at eight weeks of gestation, free T4 found in amniotic fluid, and the transfer of maternal thyroid hormones across the placenta signify the involvement of T4 hormone in fetal brain development. 

Hypothyroidism in Pregnancy

In women who suffered from thyroid dysfunction prior to pregnancy, thyroid hormonal changes and demand are magnified, leading to possible adverse pregnancy outcomes. Women with hypothyroidism have decreased fertility, and even if they do conceive, the risk of miscarriage is increased, as well as gestational hypertension, anemia, low birth weight, postpartum hemorrhage, and respiratory distress in the newborn child. 

Maternal hypothyroidism directly impacts pregnancy outcomes. The severity, timing of onset, duration, and postnatal management all influence fetal and neonatal brain development. Even mild maternal hypothyroidism may affect fetal brain development. 

Hypothyroidism, particularly subclinical, is usually asymptomatic. However, inappropriate weight gain, cold intolerance, dry skin, and delayed relaxation of deep tendon reflexes can all be indicators of hypothyroidism during pregnancy. 

Sadly, the prevalence of thyroid dysfunction in pregnant women is relatively high as thyroid dysfunction occurs in 2-3% of pregnancies and subclinical dysfunction occurs in 10% of pregnancies. Thyroid autoimmunity is even more prevalent. 

A proper functioning thyroid is critical! Selenium and other nutrients can help, but expecting mothers should also pursue complete treatment. 

How Is Selenium Involved in Pregnancy?

Selenium is one of the most important nutrients for thyroid functioning. It is important to fertility and early pregnancy since it positively impacts egg quality and supports increased thyroid hormone production and metabolism. 

One study in Italy supplemented pregnant women with an autoimmune thyroid disorder (AIT) with a specific dosage of selenium. Those who received the selenium supplement experienced a lower incidence of postpartum thyroiditis and permanent hypothyroidism. An additional study analyzed data on 1,900 participants showing an inverse relationship between serum selenium concentrations and thyroid volume, risk of goiter, and thyroid tissue damage in people with mild iodine deficiency. Lastly, in a double-blind RCT in a population with no known thyroid diseases, selenium supplementation minutely and dose-dependently affected thyroid function by decreasing serum TSH and FT4 concentrations. 

What Form of Supplementation Is Best? 

Selenium exists in two forms: inorganic (selenate and selenite) and organic (selenomethionine and selenocysteine). Both forms can be good dietary sources of selenium. Selenium is available in multivitamin supplements and as a stand-alone supplement, often in the form of selenomethionine, selenium-enriched yeast, or as sodium selenite or sodium selenate. 

Our bodies absorb more than 90% of selenomethionine, making it the most bioavailable form. 

The efficacy of selenium supplementation depends on the bioavailability of the compounds. Selenomethionine (SeMet) possesses excellent bioavailability and lower toxicity and therefore is recommended for supplemental use, particularly for long-term administration. SeMet is unique in that it incorporates into body proteins by replacing methionine. This behavior provides selenium storage in bodily tissues, making it a particularly beneficial form of nutritional supplementation. 

How Much Selenium Do I Need During Pregnancy? 

The established RDA for women is 55 mcg/day of selenium. For pregnant women, the RDA increases slightly to 60 mcg/day. 

It is documented that selenium deficiency in humans occurs when dietary intake is lower than 40 mcg/day, whereas toxicity can be observed at daily levels above 400 mcg/day. 

Am I Getting Selenium From Food? 

Selenium is an essential trace mineral that must be obtained from the diet. And yes, selenium is found in many different foods! 

However, the selenium concentration in foods fluctuates greatly by geographical location and soil makeup. This is particularly true in plant-based foods. For example, according to the US Department of Agriculture Food Composition Database, Brazil nuts have 544 mcg selenium/ounce, or 68 mcg selenium/nut. However, values vary widely from other analyses. All of this variance in plant-based food is influenced by the amount of selenium and organic matter in the soil, the form of selenium present, and soil pH. Geographic location and soil contents affect animal food products as well since animals feed on plants. However, selenium concentration in soil has a minor effect on animal products because animals maintain predictable tissue concentrations of the mineral.

Food sources high in selenium include Brazil nuts, seafood, and organ meats (kidney and liver). Other sources include meat, poultry, fish, shellfish (crab), eggs, cereals, bread (other grains), and dairy products.

Vegetables (carrots, peas, beans, potatoes, and tomatoes) contain a maximum of 6 mcg/g of selenium, and similarly, fruits rarely exceed a selenium content of 10 mcg/g. A whole egg has an average selenium content of 15 mcg/g, while a cup of milk or yogurt contains approximately 8 mcg/g of selenium. 

How Can I Tell If I Am Getting Enough Selenium?

What Do I Need to Know About Selenium Toxicity?

The Food and Nutrition Board established a Tolerable Upper Limit for both dietary and supplement selenium intake based on the amounts of selenium associated with hair and nail brittleness and loss. This limit to avoid toxicity is set at 400 mcg/day for women and pregnant women.  

Even though selenium deficiency is found worldwide in large percentages, circulating levels of this element have a narrow safety level, and toxicity can be equally as harmful. Toxicity causes significant endocrine disruptions in the synthesis of thyroid hormones and increases the risk of type 2 diabetes. Excess selenium in the body with toxic effects, known as selenosis, occurs on rare occasions but is harmful. It generally arises when repeatedly consuming selenium in excess of 400 mcg/day. 

Symptoms of selenium toxicity include nausea, vomiting, abdominal pain, diarrhea, hair loss, brittle nails, peripheral neuropathy, and the characteristic smell of garlic in sweat and breath. 

Prenatal Vitamin Brands: What's the Selenium Amount and Type in Popular Prenatal Brands?

Prenatal Analysis:

Aligned with our research on the ideal form of selenium, L-selenomethionine is in a number of the prenatal vitamins listed above. The 200 mcg amount is still significantly below the tolerable upper limit of 400 mcg. However, if dietary intake is high, this supplemental amount may not be necessary and could be harmful. If you are someone who eats multiple Brazil nuts each day, one of the prenatal vitamins without selenium may be your best choice. The other forms of selenium used in Seeking Health and Designs for Health are still safe and beneficial; the research simply indicates that those forms may not be as bioavailable as L-selenomethionine. 

Resources: 

1.     Your thyroid before, during, and after pregnancy. FullWell. Accessed May 18, 2022. https://fullwellfertility.com/blogs/blog/from-start-to-finish-your-thyroid-before-during-and-after-pregnancy

2.     Office of Dietary Supplements - Selenium. Accessed May 18, 2022. https://ods.od.nih.gov/factsheets/Selenium-HealthProfessional/

3.     Hypothyroidism in Pregnancy: Consequences to Neonatal Health | The Journal of Clinical Endocrinology & Metabolism | Oxford Academic. Accessed May 18, 2022. https://academic.oup.com/jcem/article/86/6/2349/2848391

4.     Sahay RK, Nagesh VS. Hypothyroidism in pregnancy. Indian J Endocrinol Metab. 2012;16(3):364-370. doi:10.4103/2230-8210.95667

5.     Nazarpour S, Ramezani Tehrani F, Simbar M, Azizi F. Thyroid dysfunction and pregnancy outcomes. Iran J Reprod Med. 2015;13(7):387-396.

6.     Does selenium supplementation affect thyroid function? Results from a randomized, controlled, double-blinded trial in a Danish population in: European Journal of Endocrinology Volume 172 Issue 6 (2015). Accessed May 18, 2022. https://eje.bioscientifica.com/view/journals/eje/172/6/657.xml

7.     Duntas LH. Selenium and the Thyroid: A Close-Knit Connection. J Clin Endocrinol Metab. 2010;95(12):5180-5188. doi:10.1210/jc.2010-0191

8.     Ventura M, Melo M, Carrilho F. Selenium and Thyroid Disease: From Pathophysiology to Treatment. Int J Endocrinol. 2017;2017:1297658. doi:10.1155/2017/1297658