DIO1 Activity
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.
What this measures
How your DNA shapes DIO1 activity.
DIO1 (deiodinase 1) is one of the three enzymes that activate thyroid hormone — converting T4 (the storage form) into T3 (the active form). DIO1 acts primarily in the liver and contributes to the circulating pool of T3; DIO2 (a Phase 1 exemplar) acts in tissues. Together they decide how much active thyroid hormone reaches the cells that need it.
Carriers of common DIO1 reduced-activity variants are associated with lower hepatic T3 production and reduced circulating T3 levels — even when T4 and TSH look adequate. Carriers of DIO2 reduced-activity variants compound the picture at the tissue level. The combined effect can produce the symptom pattern of clinical hypothyroidism even with "normal" thyroid labs.
Selenium is the required cofactor for both DIO1 and DIO2 — Brazil nuts, seafood, and selenium-rich foods support both enzymes. Iodine status shapes upstream T4 availability. Zinc and iron each influence the broader thyroid network. Chronic stress, severe caloric restriction, and certain medications (lithium, amiodarone) all suppress deiodinase activity.
The "labs look normal but I feel hypothyroid" pattern is much more common than the standard workup suggests. Which DIO1/DIO2 combination you carry decides whether the conversation with a clinician needs to include free T3 testing — and whether selenium and supporting cofactors are the variable that actually shifts how you feel.
DIO1 Activity is one specific finding in this system. Your Genomic Lifestyle Optimization Report shows where your variants place you on the hormones spectrum — and what you can do about it: it renders as a dark card with a color marker calibrated to your variants, opening with the gene mechanism and closing with a practical, mechanism-led recommendation.
Want to see what a real Mosaic dark card looks like? Walk through a sample report →
In context
Thyroid: the 4-insight cluster.
DIO1 Activity is one finding in a tightly-related cluster. Mosaic sequences the other 3 alongside it so you see the whole biology — not an isolated data point.
Questions people ask
About DIO1 Activity.
More from Hormones
Autoimmune Thyroid Risk
Genetic predisposition to Hashimoto's or Graves' varies widely. If you're in the higher-risk group, early antibody monitoring can catch the process years before full disease expression—when lifestyle interventions still have leverage.
Read insight →Cortisol Production
Some people run naturally high on cortisol; others trend low. Neither is inherently bad—but each benefits from different stress-management strategies. High producers need more active recovery; low producers may need adaptogen support. One-size-fits-all stress advice ignores this completely.
Read insight →Cortisol Receptor Sensitivity
Same cortisol level, very different felt experience. High receptor sensitivity means stress hits harder even when cortisol is "normal." This explains why some people thrive in chaos while others burn out—and it's not about being weak. It's about wiring.
Read insight →DIO2 Activity
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.
Read insight →Dihydrotestosterone (DHT) Levels
DHT is the more potent downstream product of testosterone. High converters may see effects on hair loss, prostate health, or skin—and often benefit from DHT-blocking strategies before symptoms become problems. Low converters rarely need to worry about it.
Read insight →Estrogen Metabolism
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.
Read insight →
One test. 108 personalized findings. All yours.
Order your Mosaic kit. Receive your raw genomic data and the full Genomic Lifestyle Optimization Report in 15–20 days.