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.