Hashimoto's Thyroiditis: How Genetic Predisposition Connects to Autoimmune Inflammation

Hashimoto's thyroiditis is the most common autoimmune endocrine disease: by various estimates, it develops in approximately 5% of the general population, with women affected 7–10 times more often than men (Cárdenas-Roldán et al., Autoimmun. Rev., 2012). But the mechanism that triggers the disease in some people and spares others remains an active area of research.

The thyroid gland here is the target organ, not the source of the failure: the primary disruption occurs in immune regulation. This shifts the question from "why isn't the gland working" to "why does the immune system attack its own tissues, and what role does genetics play".

On how the hormonal profile connects to inherited characteristics — Weight, Energy, Mood and Your DNA: How Hormones Are Linked to Genetics.

Why the Thyroid Gland Becomes a Target of Its Own Immune System

The pathogenesis of autoimmune thyroiditis (AIT) is rooted in a persistent failure of the self/non-self recognition mechanism. Under normal conditions, the immune system destroys autoreactive lymphocytes — those directed against the body's own tissues — during their maturation in the thymus. When this surveillance breaks down, through antigen mimicry or disruption of central tolerance, autoreactive cells are not eliminated and begin attacking the body's own tissues (Simmonds & Gough, Clin. Exp. Immunol., 2004).

In AIT, two thyroid proteins become the targets. Plasma cells produce antibodies against thyroid peroxidase (TPO Ab) — the enzyme responsible for synthesising thyroid hormones — and against thyroglobulin (TG Ab) — the protein in which hormones are stored within the gland tissue. The chronic accumulation of lymphocytes in the gland tissue gradually replaces functional cells with connective tissue. The result is reduced synthesis of thyroid hormones and the development of hypothyroidism.

Tolerance failure → activation of autoreactive T-lymphocytes → chronic inflammation of thyroid tissue → production of TPO Ab and TG Ab → follicle destruction → hypothyroidism

This process can unfold over years before the first laboratory signs appear: thyroid tissue has a substantial functional reserve, and TSH rises only after a significant portion of the gland has already been damaged.

Genetic Factors in AIT Predisposition

Hashimoto's thyroiditis is a multifactorial disease with a pronounced polygenic nature. There is no single "Hashimoto's gene" whose presence would definitively determine development of the condition. Instead, there is a combination of dozens of genetic variants, each of which lowers the threshold for activating an autoimmune response. No individual locus determines the disease — it merely raises predisposition in a population context (Tomer, Annu. Rev. Pathol., 2014).

GWAS studies confirmed that the most significant associations are concentrated around three systems: HLA (human leukocyte antigen) loci, T-cell regulation genes, and genes for thyroid-specific proteins (Aceves-Ávila et al., Front. Genet., 2021).

Genes of Immune Control

Certain HLA-DR3 and HLA-DR5 alleles cause antigen-presenting cells to present fragments of thyroid peroxidase as threatening objects with excessive efficiency, initiating the autoimmune cascade. Among HLA genes, the HLA-DR3 allele carries the strongest association with AIT (Aceves-Ávila et al., Front. Genet., 2021).

The gene CTLA4 (cytotoxic T-lymphocyte associated protein 4) normally acts as a regulator: once a threat has been dealt with, it brakes T-lymphocyte activation to prevent an excessive immune response. Variants of CTLA4 with reduced function weaken this brake — and the body loses the ability to switch off activated T-cells in time. Under these conditions, inflammation in the thyroid tissue becomes chronic. CTLA4 and PTPN22 (protein tyrosine phosphatase non-receptor type 22) are the non-HLA loci whose association with AIT has been most consistently confirmed in independent GWAS studies (Aceves-Ávila et al., Front. Genet., 2021).

PTPN22 encodes a lymphoid tyrosine phosphatase that controls the activation threshold of T-cell receptors. Certain variants of this gene make T-lymphocytes hypersensitive: they activate in response to minimal physiological signals that would ordinarily be ignored.

Vitamin D Receptor Variants

The gene VDR (vitamin D receptor) is an independent factor in its own right. In contemporary endocrinology, vitamin D is understood primarily as an immunomodulatory steroid hormone: by interacting with VDR on the surface of immune cells, it suppresses pro-inflammatory type-1 T-helper cells (Th1) and stimulates regulatory T-cells (Treg), which are responsible for suppressing autoimmune reactions. Polymorphisms of VDR that reduce receptor sensitivity are associated with an elevated risk of autoimmune thyroid disease, although the effects vary by specific allele and population (Feng et al., Int. J. Mol. Sci., 2025). Even when vitamin D blood levels are normal, immune cells may be in a state of functional deficiency.

T4→T3 Conversion: How Genetics Influences Function

The situation in which TSH and free T4 are within range but a person experiences the classic symptoms of tissue-level hypothyroidism is not uncommon in AIT. The explanation lies at the level of peripheral hormone conversion.

The thyroid gland synthesises predominantly the biologically inactive prohormone thyroxine (T4). To perform its function in target cells — the brain, muscles, liver — T4 must be converted into active triiodothyronine (T3). This process is carried out by deiodinases encoded by the genes DIO1 (iodothyronine deiodinase 1) and DIO2 (iodothyronine deiodinase 2).

DIO2 → local conversion of T4 → T3 in tissues (brain, pituitary, muscles)

The Thr92Ala polymorphism in the DIO2 gene reduces the activity and protein stability of the type-2 deiodinase enzyme. Carriers of this polymorphism have adequate T4 in their blood, but reduced intracellular conversion leads to a deficit of active T3 — particularly in tissues that depend on local DIO2 activity (Castagna et al., J. Clin. Endocrinol. Metab., 2017). Since TSH secretion is regulated by T4 and T3 levels in the blood rather than their concentrations in tissues, a standard blood test will show normal results while the tissues experience a deficiency of the active hormone.

Nutrient Cofactors: Iron, Selenium, Iodine, and What DNA Reveals

Whether a genetic predisposition to AIT is expressed partly depends on how efficiently the body absorbs key micronutrients. This, in turn, is linked to the genetic characteristics of each nutrient's metabolism.

Iron

Thyroid peroxidase (TPO) is a haem-dependent enzyme whose active centre contains iron and requires it for normal function. A systematic review confirmed that iron deficiency raises levels of TPO and thyroglobulin autoantibodies, and that lower ferritin is associated with reduced free thyroid hormone levels (Muñoz et al., Nutrients, 2023). Genetic variants that impair iron absorption or hepcidin regulation can amplify this effect independently of dietary intake.

Selenium

The deiodinases responsible for T4→T3 conversion are selenoproteins that contain the amino acid selenocysteine. Selenium is also a component of glutathione peroxidase — the enzyme that provides antioxidant protection for thyrocytes. During hormone synthesis, thyrocytes generate hydrogen peroxide. If, owing to genetically reduced selenium absorption, glutathione peroxidase cannot adequately neutralise this peroxide, localised oxidative stress develops and amplifies autoimmune damage. A systematic review and meta-analysis of randomised trials showed that selenium supplementation in AIT reduces TPO antibody levels (Yin et al., Thyroid, 2024), while genetic variants that reduce selenium absorption weaken the protective action of glutathione peroxidase.

Iodine

In the presence of an autoimmune process, excessive iodine intake intensifies the immune response to thyroglobulin — by increasing its recognition by autoreactive T-lymphocytes — and can aggravate inflammation. Genetic analysis evaluates individual characteristics of iodine metabolism, which a physician considers alongside laboratory findings when determining supplement requirements and dietary adjustments.

Stress and Cortisol: Why Chronic Load Is a Trigger, Not a Cause

Beyond micronutrients, chronic stress also influences the course of AIT — not as a cause, but as a trigger that amplifies an already present genetic predisposition. Sustained stress leads to chronically elevated cortisol, which suppresses the activity of DIO2 and simultaneously activates DIO3 (iodothyronine deiodinase 3), diverting T4 not towards active T3 but towards reverse triiodothyronine (rT3) (Halsall & Oddy, Ann. Clin. Biochem., 2021). Reverse T3 is biologically inert but competes with active T3 for the same receptors, blocking cellular access to the active hormone.

Chronic stress → elevated cortisol → suppression of DIO2 + activation of DIO3 → rising rT3 → competition with active T3 for receptors → functional hypothyroidism with normal TSH

In people with genetically slowed T4→T3 conversion, rT3 accumulation further deepens the tissue deficit of T3, even when the thyroid gland is still synthesising a sufficient volume of hormone. On the connection between cortisol, stress, and overall hormonal balance — Cortisol and Chronic Stress: Hormonal Mechanisms of Fatigue and Individual Differences.

What Genetic Analysis Shows and How to Use Its Results

Genetic analysis is not a tool for capturing current status. It reveals individual predispositions before irreversible tissue damage develops.

Understanding your own genetic characteristics makes it possible to build personalised preventive strategies aimed at reducing the risk of disease progression, rather than waiting for symptoms that already require treatment:

• Immune risk assessment: identifying unfavourable variants in the HLA, CTLA4, or PTPN22 loci reveals the hereditarily determined threshold of immune response activation relative to the population. An elevated risk is grounds for paying attention to control of chronic infections, toxic burden, and deficiency states.

• Individual characteristics of nutrient absorption: knowledge of the genetic characteristics of selenium and iron metabolism, and of VDR receptor sensitivity, allows a physician to prioritise monitoring targets and avoid dangerous iodine excess in the context of an ongoing autoimmune process.

• Predicting tissue response: assessment of conversion variants (DIO1/DIO2) explains why hypothyroid symptoms appear despite normal blood test results, and highlights the need for regular monitoring not only of TSH but also of free hormone fractions.

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The Genetic Profile as a Preventive Tool

Hashimoto's thyroiditis is one of those conditions in which the gap between "normal lab results" and how a person actually feels can persist for years. The polygenic nature of the disease, slowed T4→T3 conversion, and individual characteristics of micronutrient metabolism — each of these factors does not operate in isolation and does not provide a complete explanation on its own. A genetic test does not establish a diagnosis, but it maps an individual risk profile and indicates what to monitor before laboratory values move outside the reference range.

How genetics connects to hormonal system function — Hormonal Imbalance and DNA: What a Genetic Test Reveals About Your Hormonal System (Article #47).

Genetic test results are not a diagnosis and do not replace a consultation with a doctor. The Apixmed Prism report provides genetic context that complements clinical test results and supports informed decision-making together with your physician.

Sources

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5. Castagna, M. G., Dentice, M., Cantara, S., et al. (2017). DIO2 Thr92Ala Reduces Deiodinase-2 Activity and Serum-T3 Levels in Thyroid-Deficient Patients. Journal of Clinical Endocrinology & Metabolism, 102(5), 1623–1630.https://doi.org/10.1210/jc.2016-2587

6. Muñoz, M., Martín, M., Unanue, N. (2023). Relationship between Iron Deficiency and Thyroid Function: A Systematic Review and Meta-Analysis. Nutrients, 15(22), 4790.https://doi.org/10.3390/nu15224790

7. Yin, X., Ni, Q., Chen, Y., et al. (2024). Selenium Supplementation in Patients with Hashimoto Thyroiditis: A Systematic Review and Meta-Analysis of Randomized Clinical Trials. Thyroid, 34(3), 360–373.https://doi.org/10.1089/thy.2023.0505

8. Halsall, D. J., Oddy, S. (2021). Clinical and laboratory aspects of 3,3′,5′-triiodothyronine (reverse T3). Annals of Clinical Biochemistry, 58(1), 29–37.https://doi.org/10.1177/0004563220969150

9. Feng, M., Li, H., Zhang, Q., et al. (2025). Vitamin D Receptor Polymorphisms and Immunological Effects of Vitamin D in Hashimoto's Thyroiditis. International Journal of Molecular Sciences, 26(21), 10576.https://doi.org/10.3390/ijms262110576