Vol. 3 No. issue2,April2026,pg2224-2239 (2026): Exploring-Nonlinear Associations Between Urinary Iodine Concentration and Thyroid Dysfunction: A Cross-Sectional Analysis of Exposure Ranges and Within-Sample Odds

Background: Both limited and high iodine exposure may be relevant to thyroid health, but this biological possibility does not establish the shape of an association in a particular adult sample. Categorising urinary iodine concentration (UIC) alone can obscure variation within ranges, while imposing a quadratic model may produce an apparent U-shaped relationship that is poorly supported by the observations.

Objective: To specify a cross-sectional analysis of UIC ranges and adjusted odds of thyroid dysfunction, allowing the exposure–outcome relationship to be nonlinear without assuming a U-shaped pattern in advance.

Methods: An analytical study is proposed among consenting nonpregnant adults with paired urine and serum measurements and documented demographic, dietary and clinical information. Current functional status will be established using thyroid-stimulating hormone, free thyroxine, free triiodothyronine and clinical review. UIC will be examined primarily as a continuous exposure using a parsimonious restricted cubic spline within logistic regression. Prespecified concentration ranges will support secondary descriptive comparisons. Overall association and departure from linearity will be tested separately. Adjusted odds-ratio curves will be reported with confidence intervals, exposure distributions and explicit information about sparse regions. Treatment, sampling by disease group, urine dilution and missing observations will receive prespecified consideration.

Results status: No participant dataset has been supplied. This paper therefore presents the study rationale, analytical framework and reporting requirements; it does not report observed group counts, odds ratios, P values or a fitted exposure–outcome curve.

Conclusion: Flexible analysis can determine whether the available observations are compatible with a linear, nonlinear or uncertain relationship. Within-sample odds do not establish incidence, causal effects or an optimal individual iodine target. Any conclusion about shape must consider precision, sampling, treatment and support across the exposure range.

Keywords: urinary iodine concentration; thyroid dysfunction; restricted cubic splines; nonlinear association; adjusted odds ratio; cross-sectional study; iodine exposure