Hashimoto's is the most common cause of an underactive thyroid — and it's autoimmune. Your immune system slowly attacks and destroys your thyroid gland, so it makes less and less hormone. Understanding that it's an autoimmune disease, not just 'a slow thyroid,' changes how you test, treat, and manage it.
In Hashimoto's, immune cells and antibodies (against thyroid peroxidase, TPO, and thyroglobulin) infiltrate and gradually destroy the thyroid gland. As the gland fails, it makes less thyroid hormone and you become hypothyroid — slowing metabolism body-wide. It is the leading cause of hypothyroidism wherever iodine is sufficient. Evidence ●●●●●
It's far more common in women (roughly 5–10×), typically emerging between the 30s and 50s, and it clusters in families and alongside other autoimmune conditions (celiac disease, type-1 diabetes, vitiligo, pernicious anaemia). If autoimmune disease runs in your family, your odds are higher — and worth screening for. Evidence ●●●●○
The standard framing — 'your thyroid's underactive, take levothyroxine' — is true but incomplete. This is an autoimmune, systemic disease that affects energy, mood, weight, heart, cholesterol, and fertility, and many people feel there's more to manage than a single pill. That fuller picture — the antibodies, the triggers, the levers — is what this guide is about. Evidence ●●●●○
How to read the evidence dots
Throughout this guide, ●●●●● means strong, replicated evidence and ○○○○○ means essentially unproven. Thyroid autoimmunity attracts a lot of confident claims — from mainstream medicine and the wellness world alike — so the grades matter: they separate what's proven from what's promising from what's just sold.
The Hashimoto’s Guide