Like most autoimmune disease, Hashimoto's is genes loaded, then something pulls the trigger. Knowing the triggers matters, because a few are things you can influence — and one common 'health' habit can actively make it worse.
Hashimoto's runs strongly in families (genes affecting the immune system, plus being female and the role of estrogen). But genes need a trigger: pregnancy and the postpartum period, infections (Epstein-Barr virus is a leading suspect), stress, and environmental exposures can all help tip a susceptible immune system into attacking the thyroid. No single trigger is 'the cause' — it's a stack. Evidence ●●●○○
The counter-intuitive one: too much iodine can trigger or worsen Hashimoto's by fuelling the autoimmune attack — which is why high-dose iodine supplements and kelp/seaweed are a common self-inflicted mistake in people trying to 'support' their thyroid. (Severe deficiency is also harmful; it's a U-shaped curve, and the goal is adequacy, not megadosing.) Evidence ●●●○○
A scatter of associations you can act on: selenium deficiency, low vitamin D, and gut problems (Hashimoto's overlaps strongly with celiac disease, and with a leaky, dysbiotic gut). None is a proven sole cause, but several are worth checking and correcting. (Oddly, smoking associates with lower Hashimoto's risk — but it's harmful overall and worsens the related Graves' disease, so it's no strategy.) Evidence ●●●○○
Don't take high-dose iodine or kelp
Taking iodine or kelp to 'support' the thyroid is a common mistake in Hashimoto's — excess iodine can actively worsen the autoimmune attack, the opposite of what you want. Aim for dietary adequacy, and leave the megadoses alone.
The Hashimoto’s Guide