I have translated some of Thyroid Patients Canada’ articles on conversion into Norwegian. These articles are very interesting, esp this one, https://thyroidpatients.ca/2019/11/12/the-basics-of-thyroid-hormone-action-transport-and-conversion/
Most of the hormone conversion, T4 to T3 happens outside the thyroid. though there are big individual differences how much. There are 3 enzymes responsible for this conversion, D1, D2 and D3. They do a lot more than convert T4 to T3 though.
You find the part on conversion in the latter part of the article. Here Tania describes how D2, D2 and D3 do their various jobs in the different tissues. It is very interesting as well, that certain medicines like glucocorticoids, like prednisone og hydrocortison, and beta blockers. and calorie restricted diets all impact conversion negatively.
You might feel a little cheated, to click in and this is all you get. But hopefully I point you in a direction you wouldn’t have found otherwise. I translate articles and studies I find interesting into Norwegian, for people in Scandinavia to read. So I link to the article, and just wet your appetite so to speak.
This is a simplified figure of how the deiodinases work. They do a lot more than this, so read that article!:)