Phase 1 · Understand the Body — Why the disease behaves the way it does.
The One-Line Truth
The whole disease in one chain
Everything on this page exists to explain this line.
Your own immune system slowly destroys your thyroid gland, so the gland makes less and less thyroid hormone over time.
The Immune System Turns on the Factory
Your thyroid is like a hormone factory, and thyroid peroxidase (TPO) is the key machine on the assembly line that builds the product (T4 and T3 hormone). Normally, the factory's security guards (your immune system) protect the machinery and only attack invaders. But in Hashimoto, those security guards get confused and start smashing the TPO machine with hammers and fire, thinking it is an enemy. As the machine breaks down piece by piece, the factory produces fewer and fewer hormone packages. The boss upstairs (your pituitary) sees the drop in products leaving the factory and yells louder and louder into the megaphone (TSH), trying to get the broken factory to work faster. But no amount of yelling fixes a broken machine—the factory just keeps shrinking and producing less, while the boss's frantic TSH calls climb higher and higher, unanswered.
American Thyroid Association (ATA) Guidelines for Hypothyroidism 2014 · Endocrine Society Clinical Practice Guideline on Hypothyroidism 2012 · UpToDate evidence summary on Hashimoto thyroiditis · Cochrane systematic reviews on thyroid disease · reviewed 2026-07-07