Oral oestrogen increases the need for thyroxine, going on the COP is what led to me becoming first very ill with hypothyroidism and then much later diagnosed when I was 20 and in a cfs like state.
I suspect there was some early illness going on beforehand. I remember having awful bouts of sweatiness in the year or two before. But who knows.
I think I read once that goitregens in animal feed have caused thyroid issues, and something to do with soy.
Pregnancy hormones can affect the thyroid; you need more thyroxine during pregnancy for baby and a healthy thyroid actually grows a little.
I've avoided tofu etc but actually just starting to think I'll try to see if it helps. From lunch onwards.
They probably* haven't researched it properly.
*definately.
But the answer is to be monitored and raise if needed. I think it's more of an issue if it's causing subclinical issues.
Apparently an obscure study tracked ovulating women on thyroxine and found tsh rises during ovulation for example.
I'm interested in all this as I was sooo badly handled when first (not)diagnosed and then again when pregnant etc. And my Gp recently said general practice isn't very good on thyroid stuff.