Hashimoto's thyroiditis (HT) is the most common autoimmune thyroid disease and is increasingly recognized as a condition associated with chronic low-grade systemic inflammation beyond thyroid hormone dysfunction. The soluble α-Klotho-fibroblast growth factor 23 (FGF23) axis plays a central role in mineral metabolism, inflammation, and aging-related pathways; however, its involvement in HT has not been adequately characterized.

This prospective case-control study aims to compare serum soluble α-Klotho and FGF23 levels between adults with Hashimoto's thyroiditis and age- and sex-matched healthy controls, and to investigate their associations with thyroid function parameters, inflammatory markers, and autoimmune burden. In addition, the FGF23/sKlotho ratio will be evaluated as an integrated marker of functional imbalance within the Klotho-FGF23 axis.

The study seeks to determine whether Hashimoto's thyroiditis is associated with systemic dysregulation of the Klotho-FGF23 pathway independent of renal function and mineral metabolism.

Hashimoto's thyroiditis (HT) is a chronic autoimmune disease characterized by lymphocytic infiltration of the thyroid gland and the presence of anti-thyroid peroxidase (anti-TPO) and anti-thyroglobulin (anti-TG) antibodies. While traditionally defined by thyroid hormone dysfunction, emerging evidence suggests that HT is associated with persistent immune activation, oxidative stress, and low-grade systemic inflammation.

The α-Klotho-FGF23 axis is an endocrine pathway involved in phosphate metabolism, vitamin D regulation, inflammation, and aging-related biological processes. Soluble α-Klotho (sKlotho) exerts anti-inflammatory and anti-oxidative effects, whereas FGF23 is increasingly recognized as a hormone influenced by inflammatory stimuli. Dysregulation of this axis has been reported in various chronic inflammatory conditions; however, data in autoimmune thyroid disease are limited.

This prospective, single-center case-control study will include adult patients with Hashimoto's thyroiditis and healthy controls without thyroid or systemic autoimmune disease. Serum soluble α-Klotho and FGF23 levels will be measured using enzyme-linked immunosorbent assay (ELISA). Clinical data including thyroid function tests (TSH, free T4), thyroid autoantibodies (anti-TPO, anti-TG), renal function parameters, mineral metabolism markers (calcium, phosphorus, 25-hydroxyvitamin D), and C-reactive protein (CRP) will be recorded.

Primary analyses will compare sKlotho and FGF23 levels between groups. Secondary analyses will evaluate associations between these biomarkers and autoimmune burden (anti-TG levels), inflammatory status (CRP), and thyroid function. Multivariable regression models will be used to determine independent associations after adjustment for potential confounders.

The study also aims to evaluate the FGF23/sKlotho ratio as an integrated indicator of functional imbalance within the Klotho-FGF23 axis in Hashimoto's thyroiditis.

Inclusion Criteria:

Adults aged 18-65 years

Ability to provide written informed consent

Hashimoto group: Diagnosis of Hashimoto's thyroiditis based on positive anti-thyroid peroxidase (anti-TPO) and/or anti-thyroglobulin (anti-TG) antibodies with compatible clinical/ultrasound findings (as available)

Control group: No history of thyroid disease or autoimmune disease; normal thyroid function tests (TSH and free T4 within reference range)

Exclusion Criteria:

Pregnancy or breastfeeding

Known chronic kidney disease (eGFR < 60 mL/min/1.73 m²)

Active infection or acute inflammatory condition within the past 4 weeks

Known malignancy under active treatment

Chronic inflammatory/autoimmune disease other than Hashimoto's thyroiditis (e.g., rheumatoid arthritis, SLE, IBD)

Use of systemic glucocorticoids or immunosuppressive therapy within the past 3 months

Known parathyroid disease or disorders of calcium/phosphate metabolism

Current use of medications strongly affecting mineral metabolism (e.g., phosphate binders, active vitamin D analogs)

Adult participants aged 18-65 years will be recruited from the endocrinology outpatient clinic of a tertiary care center. The study population will consist of patients diagnosed with Hashimoto's thyroiditis and age- and sex-matched healthy controls without thyroid or systemic autoimmune disease. All participants will undergo baseline clinical evaluation and venous blood sampling for measurement of soluble α-Klotho and FGF23 levels, along with routine laboratory parameters.