Selenium is crucial for the local production of thyroid hormone T3 in tissues. However, for example, consuming shellfish or seafood, halibut, cod, or flounder can provide enough selenium to maintain a normal supply of thyroid hormones in tissues. Eating a hearty seafood meal once a week can supply the necessary selenium. Selenium helps your tissues synthesize thyroid hormones. Your blood circulation may have enough T4, but without selenium, different tissues cannot produce the T3 they need, such as the T3 required for heart metabolism. Selenium is essential for the thyroid to function properly in the deiodinase (thyroid hormone conversion) system. Selenium is necessary to activate the thyroid, and ultimately other B vitamins and all nutrients are also indispensable. But the most common issues are hypothyroidism and selenium deficiency. To activate the thyroid with selenium, glucose is needed. If your diet lacks selenium or nutrients like vitamin D necessary for normal thyroid function, T4 can interfere with T3's action… If you are deficient in selenium and cannot convert it to T4, selenium will accumulate over a few weeks, eventually interfering with T3 and producing anti-thyroid effects. I believe 100 micrograms is a safe supplemental dose. — Ray Pete