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 hormone health spectrum — and what you can do about it.
In your report
Where DIO1 Activity lives inside your Genomic Lifestyle Optimization Report.
DIO1 Activity renders as a dark-background card with a color marker calibrated to your specific variants. The card opens with the gene mechanism, shows your result at a glance via that marker, and closes with a practical, mechanism-led recommendation — what to eat, what to time, what cofactors to support.
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.
- Autoimmune Thyroid RiskGenetic 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.
- DIO2 ActivityDIO2 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.
- Thyroid Stimulating Hormone (TSH) LevelsYour individual optimal TSH is often narrower than the lab reference range. Someone who feels best at 1.0 will feel terrible at 3.5—even though both are "normal." Knowing your sweet spot ends years of being told "you're fine" when you're clearly not.
Questions people ask
About DIO1 Activity.
- How does my DNA influence dio1 activity?
- The DIO2 thyroid-conversion enzyme that turns inactive T4 into active T3. The COMT and CYP1A1 enzymes that move estrogen through Phase I detox. The SHBG variants that bind or release testosterone. The CYP17A1 and HSD3B2 nodes in the steroid synthesis cascade. Your "normal" labs may sit on top of a genome that needs a different intervention.
- What kind of test do I need to see my DIO1 Activity result?
- Whole-genome sequencing at 30× clinical depth. Consumer SNP-chip tests like 23andMe or AncestryDNA only read ~0.02% of your DNA and miss most of the variants this analysis needs. Mosaic reads all 3 billion base pairs and produces the full 108-insight report.
- How is DIO1 Activity different from clinical lab testing?
- Clinical labs measure downstream biomarkers — blood levels, hormone values, metabolic byproducts — at a single point in time. Genomic insights like DIO1 Activity reveal the underlying variant that shapes the biology, which is constant for life. The two are complementary: labs show the current snapshot; genomics shows the long-term tendency and where lifestyle leverage is highest.
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