Selenium

Selenium is an essential trace mineral whose primary physiological role, in Ray Peat's framework, is enabling the conversion of the storage thyroid hormone thyroxine (T4) into the active metabolic hormone triiodothyronine (T3). 2 Peat argued that selenium is essential for the local production of T3 in the tissue, making it a critical cofactor for maintaining a high metabolic rate. 5 A deficiency in selenium directly impairs this conversion, leading to an accumulation of inactive T4 and symptoms of hypothyroidism, even when the thyroid gland itself is producing hormones. 2

Peat consistently recommended obtaining selenium from whole food sources, specifically seafood, rather than relying primarily on supplements. 10 He noted that anything growing in the ocean has access to selenium, iodine, and other trace minerals, whereas land-grown foods are often deficient due to soil depletion. 7 He advised consuming marine foods such as shrimp, oysters, cod, sole, or halibut about once or twice a week to provide adequate selenium, along with beneficial copper and iodine. 3 This preference for seafood was partly due to his caution regarding certain supplement forms; he considered sodium selenite to be pro-oxidative and potentially unsafe. 10

When supplementation was necessary, Peat viewed it as a safe and effective short-term intervention to restore thyroid function. He stated that 100 micrograms of selenium is a safe supplemental amount. 5 For correcting a deficiency that impairs T4 conversion, he suggested a loading phase of 200 mcg daily for the first week, followed by a maintenance dose of once or twice a week. 1 He specifically recommended selenium in conjunction with progesterone to help restore thyroid and liver functions when they are suppressed by factors like excess iodine or estrogen. 3 The liver is the primary site of T4 to T3 conversion, a process that requires not only selenium but also adequate glucose, which is why Peat linked selenium's effectiveness to a diet sufficient in carbohydrates. 6

The importance of selenium extends to mitigating the toxic effects of other dietary and environmental factors. Peat explained that the protective nutrients in cod liver oil, vitamins A and D, are so crucial that they partially offset the damage from the polyunsaturated fats (PUFAs) also present in the oil. 2 Similarly, the selenium naturally found in seafood provides a crucial safeguard against the heavy metals like mercury that accumulate in larger, older fish. 2 This protective role is part of why Peat considered occasional seafood consumption a valuable practice for maintaining overall metabolic health and resilience against stress. 2