Are you on medication for it? I would mention it because insurance companies will do anything to wriggle out of paying up and not declaring a medical condition would be one of their reasons. Having said that, I have a friend who has been on medication for it for years and has never declared it. I doubt it would make any difference to your premium TBH
I'd just declare it then. Not many insurance companies let you exclude medical conditions these days but Insureandgo do. So you could complete their medical questionnaire then exclude that condition - that's assuming you don't have any other conditions because you have to either cover them all or exclude them all.
Mrs G has a similar problem and has been taking Throxine for many year. When we renew our travel insurance they ask if we want this condition to be included in the cover, no is the answer, and therefore anything related to the thyroid is not included in the cover.
If you don't mention it, and you need to claim for something else and it comes out that you are on prescribed medication, it could invalidate your insurance.
I also have an under active thyroid , and take meds for it, and I always mention it on all travel ins. forms. NEVER omit anything. Wherever you're going Salad, have a great time. I'm envious. xx