Thyroid Autoimmunity, Thyroid Functions, and Metabolic Risk in Nonfunctional Adrenal Adenomas: A Case-Control Study

Technical Summary: Thyroid Autoimmunity, Thyroid Functions, and Metabolic Risk in Nonfunctional Adrenal Adenomas

Problem Statement
Non-functioning adrenal incidentalomas (NFAI) are adrenal masses discovered incidentally that are clinically and biochemically inactive. While historically considered "silent," emerging evidence suggests these patients face elevated metabolic and cardiovascular risks, potentially driven by low-grade chronic inflammation or subclinical steroid activity. The relationship between NFAI and thyroid autoimmunity remains unclear; existing literature is limited and yields conflicting results regarding the frequency of thyroid autoantibodies and thyroid function in these patients. Furthermore, the potential interplay between adrenal hormones (specifically aldosterone) and thyroid autoimmunity in euthyroid NFAI patients has not been thoroughly investigated.

Methodology
This study employed a cross-sectional, matched case-control design conducted at a tertiary healthcare center in Bursa, Turkey, between September 2024 and September 2025.

• Participants: The study included 72 euthyroid NFAI patients and 72 healthy controls. Groups were matched for age, gender, and Body Mass Index (BMI).
• Inclusion/Exclusion: Participants were aged 18–65. Exclusion criteria included pregnancy, liver/kidney failure, malignancy history, uncontrolled diabetes (HbA1c > 7.5% or insulin use), resistant hypertension, and use of mineralocorticoid receptor antagonists.
• Diagnostic Protocol: NFAI diagnosis was confirmed via contrast-enhanced CT (excluding lesions with Hounsfield Units ≥10 or diameter >4 cm) and rigorous hormonal exclusion. Functional adenomas were ruled out using a 1 mg overnight dexamethasone suppression test (DST), 24-hour urine metanephrines/normetanephrines, and plasma aldosterone/renin activity assessments.
• Measurements:
  • Thyroid: TSH, free T4 (fT4), free T3 (fT3), Anti-thyroid peroxidase (Anti-TPO), and Anti-thyroglobulin (Anti-Tg) antibodies.
  • Metabolic: Fasting glucose, insulin, lipid profile (Total, LDL, HDL, Triglycerides), HOMA-IR, and non-HDL cholesterol.
  • Adrenal: Cortisol (post-DST), aldosterone, renin, and metanephrines.
• Statistical Analysis: Non-parametric tests (Mann–Whitney U, Chi-square/Fisher exact) and Spearman correlation analyses were performed using SPSS v22.0. A p-value < 0.05 was considered significant.

Key Results

• Demographics: No significant differences were found between groups regarding age (p=0.884) or BMI (p=0.08).
• Thyroid Function: Free T4 levels were significantly lower in the NFAI group compared to controls (median 1.0 vs. 1.1 ng/dL; p=0.002), despite both groups being euthyroid. TSH and fT3 levels showed no significant difference.
• Thyroid Autoimmunity: The prevalence of thyroid autoimmunity (Anti-TPO and/or Anti-Tg positivity) did not differ significantly between NFAI patients and healthy controls.
• Metabolic Risk: NFAI patients exhibited significantly higher fasting glucose (p < 0.001) and non-HDL cholesterol (p = 0.049) compared to controls. No significant differences were observed in HOMA-IR, triglycerides, or other standard lipid parameters.
• Aldosterone and Autoimmunity Correlation: Among NFAI patients, those with autoimmune thyroid disease had significantly higher aldosterone levels than those without (p = 0.043). A moderate, statistically significant positive correlation was identified between Anti-TPO levels and aldosterone (r = 0.35; p < 0.001). No significant correlation was found between Anti-Tg and aldosterone (p = 0.06).

Key Contributions

1. Thyroid Function in Euthyroid NFAI: The study identifies that even in euthyroid NFAI patients, free T4 levels tend to be in the low-normal range compared to matched healthy controls, suggesting subtle thyroid axis alterations.
2. Metabolic Profiling: The research highlights that NFAI patients carry elevated metabolic risks, specifically regarding fasting glucose and non-HDL cholesterol, even when matched for BMI. It underscores the utility of non-HDL cholesterol as a sensitive marker for atherogenic risk in this population.
3. Aldosterone-Autoimmunity Link: The study provides novel observational data suggesting an association between thyroid autoimmunity (specifically Anti-TPO) and aldosterone levels within the NFAI population, a relationship not previously well-documented in non-hyperaldosteronism cases.
4. Methodological Rigor: By strictly excluding patients with overt hormonal excess and matching controls for BMI, the study isolates the specific metabolic and immunological characteristics of NFAI, reducing confounding variables common in previous literature.

Significance and Claims
The authors conclude that while the primary hypothesis—that NFAI patients have a higher frequency of thyroid autoimmunity than controls—was not supported, the secondary findings are clinically significant. The study posits that NFAI should not be viewed solely as hormonally silent masses but as conditions associated with increased metabolic and cardiovascular risks.

The paper claims that the observed low-normal fT4 levels and elevated non-HDL cholesterol indicate a need for broader monitoring in NFAI patients beyond standard hormonal activity checks. Furthermore, the observed correlation between Anti-TPO and aldosterone warrants further investigation into shared inflammatory or endocrine-disrupting mechanisms, though the authors explicitly state these findings represent an observational association rather than a proven causal relationship. The study suggests that the coexistence of NFAI, metabolic dysregulation, and thyroid autoimmunity may reflect a common systemic immune-inflammatory pathophysiology.