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.
What this measures
How your DNA shapes DIO2 activity.
DIO2 codes for the enzyme that converts the stored form of thyroid hormone (T4) into the active form (T3) your tissues actually use. Standard thyroid labs typically measure T4 and TSH; DIO2 efficiency happens downstream of both, and isn’t directly tested in a routine panel.
Common DIO2 variants are associated with anywhere from typical T3 production to reduced conversion efficiency. Carriers of the reduced-activity variant may experience the symptom pattern that gets called "hypothyroid" — low energy, cold intolerance, brain fog — even when serum TSH and T4 read inside the reference range.
DIO2 depends on selenium as a cofactor — Brazil nuts, fish, and brassicas all carry it. Iodine status, zinc, and the gut microbiome shape upstream T4 availability. Chronic stress slows the conversion. Each of these is a lever that influences how much active T3 actually reaches your tissues, regardless of what your TSH says.
The "labs are normal but I feel hypothyroid" pattern is common — and it’s often where DIO2 lives. Knowing your variant changes which markers matter beyond TSH, and what conversation to have with a provider about how to read your panels.
DIO2 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.
DIO2 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 DIO2 Activity.
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