The addition of iodine to tyrosine residues on the thyroglobulin protein is catalyzed by a heme-containing enzyme called TPO which utilizes H2O2 in the oxidation reaction.
What that means in English…slapping iodines on the backbone of thyroid hormone requires an enzyme that needs iron (heme) and involves playing with fire (H202 = Free radical). Thus, anemia or low iron and low thyroid hormone status is an absolute mess that is going to spin out into every system.
Combine the above mess with chronic inflammation which dysregulates pretty much every step of thyroid synthesis/conversion and a beat up antioxidant system and you are going to induce a tidal wave of compensations over time.
The antioxidant system needs adequate vitamin C and E, and also zinc, copper, manganese, iron (again), selenium, magnesium, and amino acids (protein) to synthesize glutathione and then a bunch of the former to recycle it effectively.
So if you are flippantly vegan and low on damn near every nutrient that humans need to spin these biochemical cycles, it isn’t a matter of if you are going to have problems, it is just a question of when they will become so bad that you notice them.
The thing is you don’t need any fancy expensive nutrient tests (that probably don’t measure anything of worth anyways) to act and you don’t have to be vegan or vegetarian to be low in any of the above nutrients.
To start, just put your average dietary intake into chronometer and see what happens.