DIO2: the polymorphism that sometimes complicates thyroid balance
A terrain we meet regularly in practice
As naturopaths, we frequently see people on Levothyroxine whose thyroid panel looks 'normal' on paper (TSH within range), yet who keep reporting fatigue, brain fog or concentration difficulties. This scenario comes up often enough in our practice that we wanted to dig into what research says about one of its possible explanations: the DIO2 polymorphism.
Role of DIO2
The role of DIO2 in thyroid hormone conversion
The thyroid mainly produces T4 (thyroxine), a weakly active form. To become fully functional, it has to be converted into T3 (triiodothyronine), the active form that actually acts on cellular metabolism. This conversion relies on enzymes called deiodinases, including type 2 deiodinase (D2, encoded by the DIO2 gene).
In the euthyroid state, D2 accounts for about 70% of circulating T3, the rest coming from another enzyme (D1). D2 plays a particularly important role in the brain, where it drives local conversion of T4 into T3 directly in certain tissues, notably the hypothalamus and pituitary, key hormonal regulation hubs.
Key takeaways
D2
D2 enzyme accounts for about 70% of circulating T3 in euthyroid state, and drives local T4 → T3 conversion in the brain.
Bianco AC, Endocr Rev 2002
Thr92Ala
13 to 15% of the population is homozygous for this variant (rs225014), often with reduced D2 activity.
Mentuccia, Diabetes 2002
Levothyroxine
5 to 15% of patients on LT4 monotherapy remain dissatisfied despite normalised TSH.
Wiersinga, Nat Rev Endocrinol 2014
LT4 + LT3
Several RCTs show improved quality of life in DIO2 carriers poorly controlled on LT4 alone.
Panicker, JCEM 2009
Beyond thyroid
Variant also linked, in some studies, to poorer glycaemic control in T2D and higher BMI.
Dora, EJE 2010
Polymorphism
The Thr92Ala polymorphism
The most studied DIO2 variant is the Thr92Ala polymorphism (rs225014), present in 13 to 15% of the population in the homozygous state (two copies of the variant). Several clinical studies suggest reduced D2 enzyme activity in carriers, although the exact magnitude of this effect remains debated: some studies confirm it clearly, others find no clear association with reduced tissue T3.
Mechanistic work has clarified where the issue may lie: in carriers, the D2 enzyme appears to accumulate abnormally in a different cellular compartment (the Golgi apparatus rather than the endoplasmic reticulum where it usually sits), which could disrupt cellular balance and favour localised oxidative stress.
Clinical implications
Why some people on Levothyroxine remain symptomatic
This is where the polymorphism takes on its full clinical relevance. Standard hypothyroidism treatment relies on Levothyroxine (synthetic LT4), which the body then has to convert into active T3, notably via D2. In a thyroidectomised or hypothyroid person carrying the Thr92Ala variant, this conversion may be insufficient, even once TSH normalises on treatment.
Several studies in thyroidectomised patients confirm that Thr92Ala carriers face a higher risk of reduced free T3 on LT4 monotherapy, despite TSH within range. This is one of the explanations put forward for why a fraction of patients on Levothyroxine (about 5 to 15% across studies) remain dissatisfied with their treatment and keep experiencing hypothyroid symptoms.
Randomised clinical trials have explored the value of LT4/LT3 combination therapy (Levothyroxine + Liothyronine) in these patients. Several show improved quality of life, and sometimes cognition, particularly in DIO2 carriers poorly controlled on LT4 alone. Caution is warranted, though: major endocrinology societies still recommend LT4 monotherapy as first line, citing evidence judged not yet robust enough at scale and potential cardiovascular risks linked to synthetic T3, which call for close medical monitoring.
Key point. The decision to introduce LT4/LT3 combination therapy is the sole responsibility of the treating physician or endocrinologist, given the cardiovascular monitoring it requires.
Cognition & associated terrains
DIO2, cognition, and other associated terrains
The link between DIO2 and cognitive function remains an active but unresolved research topic. A cross-sectional study in people over 65 did not find a significant association between the Thr92Ala variant and cognitive performance measured by standardised tests. Conversely, work in mice carrying the equivalent variant paints a more telling picture: localised hypothyroidism in certain brain regions, reduced spontaneous physical activity, increased sleep, and object-memory deficits, all improving with T3 supplementation and worsening when primary hypothyroidism is added. While not directly transposable to humans, these results outline a biologically plausible mechanism that justifies caution rather than outright scepticism.
Beyond cognition, this polymorphism has also been associated, in some studies, with poorer glycaemic control in people with type 2 diabetes and a higher body mass index, making it a terrain worth monitoring beyond the thyroid question alone.
Bottom line
The DIO2 Thr92Ala polymorphism is neither trivial nor systematically symptomatic: clinical penetrance varies between individuals, and the literature remains divided on the real magnitude of its impact, especially on cognition. What stands out most solidly:
Potentially reduced T4 → T3 conversion, particularly in the brain, in homozygous carriers (13 to 15% of the population).
A plausible explanation for persistent hypothyroid symptoms despite normalised TSH on Levothyroxine alone.
A studied therapeutic option (LT4/LT3 combination), to discuss only with a physician or endocrinologist, given the cardiovascular monitoring it requires.
One genetic terrain among others to know about, rather than a fatality or a single explanation.
👉 If you are on thyroid treatment and remain symptomatic despite a normal TSH, this variant can be one of the tracks to explore with your doctor. A naturopath can support you alongside, helping adapt your lifestyle to this terrain in coordination with your medical follow-up.
COMT (dopamine, working memory, stress) and FADS1/FADS2 (ALA → EPA → DHA conversion) will be covered in dedicated explorations.
Test your knowledge
Question 1 / 6
What is the role of the D2 enzyme, encoded by the DIO2 gene?
This exploration is educational. It does not replace medical advice. Any change to a thyroid treatment is the sole responsibility of your physician or endocrinologist. Terms and conditions →
Sources
Bianco AC. et al.. « Biochemistry, cellular and molecular biology, and physiological roles of the iodothyronine selenodeiodinases ». Endocrine Reviews, 2002.
Mentuccia D. et al.. « Association between a novel variant of the human type 2 deiodinase gene Thr92Ala and insulin resistance ». Diabetes, 2002.
Canani LH. et al.. « The type 2 deiodinase Thr92Ala polymorphism is associated with decreased enzymatic velocity and increased insulin resistance ». J Clin Endocrinol Metab, 2005.
Panicker V. et al.. « Common variation in the DIO2 gene predicts baseline psychological well-being and response to combination thyroxine plus triiodothyronine therapy ». J Clin Endocrinol Metab, 2009.
Wiersinga WM. et al.. « Paradigm shifts in thyroid hormone replacement therapies for hypothyroidism ». Nature Reviews Endocrinology, 2014.
Jonklaas J. et al.. « Guidelines for the treatment of hypothyroidism (American Thyroid Association) ». Thyroid, 2014.
McAninch EA. et al.. « Prevalent polymorphism in thyroid hormone-activating enzyme leaves a genetic fingerprint that underlies associated clinical syndromes ». J Clin Endocrinol Metab, 2015.
Dora JM. et al.. « Association of the type 2 deiodinase Thr92Ala polymorphism with type 2 diabetes ». European Journal of Endocrinology, 2010.
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