DIO1 handles the body-wide conversion of inactive T4 into active T3. Reduced activity is a common reason for persistent hypothyroid symptoms—fatigue, weight gain, cold intolerance—even when TSH looks "normal." Standard testing misses this entirely.

DIO2 produces T3 specifically in the brain and muscles. Low activity is one of the most common causes of brain fog and cold hands on standard thyroid treatment that looks fine on paper. If your labs are normal but you still feel terrible, this is often why.

Your liver processes estrogen through multiple pathways—some produce protective metabolites, others produce more aggressive ones. This ratio matters for long-term health and explains why some people feel great on the same hormone levels that cause problems for others. Cruciferous vegetables and targeted supplements can shift the balance.

Same estrogen level, dramatically different response. High receptor sensitivity often underlies severe PMS, difficult perimenopause, or intense menopausal symptoms. It's not in your head—it's in your receptors.