Selenium Deficiency Symptoms: 12 Signs & Thyroid Connection

Medically reviewed by Dr. Sarah Mitchell, MD — Internal Medicine

See also: Best Selenium Supplements 2026 | Magnesium Deficiency Symptoms | Best Supplements for Thyroid 2026

How Common Is Selenium Deficiency?

Selenium deficiency affects an estimated 500 million to 1 billion people worldwide.¹ In the US, most people get enough from diet, but vegetarians, people with Crohn’s disease, and those on dialysis are at risk.²

The 12 Selenium Deficiency Symptoms

Thyroid-Related

1. Hypothyroid symptoms (fatigue, weight gain, cold intolerance) Selenium is required for the deiodinase enzymes that convert T4 to active T3. Deficiency impairs thyroid hormone activation.³

2. Hashimoto’s thyroiditis flare-ups Selenium reduces thyroid peroxidase (TPO) antibodies. Deficiency worsens autoimmune thyroid disease.⁴

3. Goiter (thyroid enlargement) Combined iodine and selenium deficiency increases goiter risk.⁵

Immune & Infection

4. Frequent infections Selenium is required for glutathione peroxidase — a key antioxidant enzyme in immune cells.⁶

5. Slow wound healing Selenium deficiency impairs immune response and tissue repair.⁷

Reproductive

6. Male infertility Selenium is required for sperm motility and testosterone synthesis. Deficiency reduces sperm quality.⁸

7. Miscarriage risk Low selenium is associated with increased miscarriage risk, especially in women with thyroid antibodies.⁹

Skin, Hair & Nails

8. Hair loss Selenium is required for hair follicle cell division. Both deficiency AND excess cause hair loss.¹⁰

9. White nail spots Selenium deficiency can cause leukonychia (white spots on nails).¹¹

10. Skin discoloration Severe deficiency can cause skin and nail discoloration.¹²

Other

11. Brain fog and cognitive decline Selenium is required for brain glutathione peroxidase. Deficiency increases oxidative stress in the brain.¹³

12. Muscle weakness Selenium deficiency can cause myopathy (muscle disease), especially in severe cases.¹⁴

How to Test for Selenium Deficiency

Serum selenium: Normal range 70-150 mcg/L. Optimal: >100 mcg/L.¹⁵

Better indicator:

• Glutathione peroxidase activity — functional measure of selenium status
• Selenoprotein P — the most sensitive marker

How to Fix Selenium Deficiency

Step 1: Diet

• Brazil nuts: 1-2 nuts/day provides 100-200mcg (the richest source)
• Tuna: 90mcg per 3oz
• Eggs: 15mcg per egg
• Sunflower seeds: 20mcg per oz

Step 2: Supplement

• Selenomethionine or selenium yeast: best absorbed
• Dose: 100-200mcg/day
• Do NOT exceed 400mcg/day (toxicity risk)¹⁶

Step 3: Re-test

• Re-test serum selenium after 3 months
• Target: 100-150 mcg/L

Who Should NOT Supplement Selenium

• People with selenium levels already in the upper range
• Those on chemotherapy (selenium may interact)
• People with a history of skin cancer (high-dose selenium may increase risk)¹⁷

FAQ

How much selenium is too much? Upper limit: 400mcg/day. Toxicity (selenosis) causes hair loss, nail brittleness, and GI symptoms.¹⁸

Can selenium help with Hashimoto’s? Yes — 200mcg/day reduces TPO antibodies and improves thyroid ultrasound appearance.¹⁹

How many Brazil nuts per day? 1-2 nuts provides 100-200mcg selenium. Don’t eat more than 3-4/day (selenium toxicity risk).

Sources

1. Shreenath AP, et al. StatPearls. 2023.
2. NIH Selenium Fact Sheet
3. Köhrle J. Nat Rev Endocrinol. 2013;9(3):160-171.
4. Toulis KA, et al. Thyroid. 2010;20(10):1163-1173.
5. Selenium and Goiter, NIH
6. Selenium and Immunity, NIH
7. Selenium and Wound Healing, NIH
8. Moslemi MK, et al. J Urol. 2011;185(3):1042-1047.
9. Selenium and Pregnancy, NIH
10. Selenium and Hair, NIH
11. Selenium and Nails, NIH
12. Selenium and Skin, NIH
13. Selenium and Cognition, NIH
14. Selenium Myopathy, NIH
15. Selenium Testing, NIH
16. Selenium Upper Limit, NIH
17. Selenium and Cancer, NIH
18. Selenium Toxicity, NIH
19. Selenium and Hashimoto’s, Thyroid