Selenium is a very dangerous as a supplement

As a routine part of physicals MDs appropriately ask whether selenium has ever been supplemented. Asking about supplementation with selenium would be particularly key where there are any symptoms of hypothyroidism. Selenium should not ever by supplemented in more than around RDA dosages.

Three iodothyronine deiodinases which activate and deactivate thyroid hormones contain selenium. Taking selenium long term can result in dysregulation of selenium deiodinases.

Difficulties due to dyrsregulation of selenium deiodinases in livers do not correct with discontinuation of supplementation with selenium where megadosages of vitamin C and/or acidic drinks have been drunk which is almost all drinks. See this post.

Thyroid tests can be normal where selenium deiodinases have been dysregulated by selenium. A top  symptom of  selenium deiodinases being dysregulated  is fatigue.

Levothyroxine (T4) is a prohormone. Taking T4 is ineffective where selenium has been taken long term.

Triiodothyronine (T3) is active thyroid hormone. T3 has a half-life of 2.5 days.  T3 reacts with almost anything decreasing absorptions of T3. T3 is taken a sole time a day on nearly empty stomachs.