Thyroid Function and Glycemic Profile in Adults with Abnormal Thyroid Function Tests and a Euthyroid Comparison Group: A Cross-Sectional Study from Jazan, Saudi Arabia

Background and Objectives: Thyroid dysfunction and dysglycemia often coexist, but cross-sectional associations may reflect the populations sampled rather than a graded biochemical relationship. We compared the glycemic profile of adults with abnormal thyroid function tests with that of a euthyroid comparison group and examined whether thyrotropin (TSH), free thyroxine (FT4) and free triiodothyronine (FT3) were associated with glycated hemoglobin (HbA1c) and fasting plasma glucose (FPG) within groups. Materials and Methods: This cross-sectional study included 331 adults from Jazan, Saudi Arabia: 230 with abnormal thyroid function tests from three hospital laboratories and 101 patients selected for normal thyroid function tests and normal glucose values. Participants were classified by TSH as euthyroid (n = 102), hyperthyroid pattern (n = 56) or hypothyroid pattern (n = 173). Spearman correlations, Kruskal–Wallis tests and robust linear regression were used, with multiple-imputation, threshold, restriction and sex-stratified sensitivity analyses. Results: In the whole cohort, TSH correlated with HbA1c (ρ = 0.59) and FPG (ρ = 0.63; both p < 0.001); median HbA1c was 5.1%, 5.8% and 6.6% in the euthyroid, hyperthyroid pattern and hypothyroid pattern groups. Within the hypothyroid pattern group, TSH was unrelated to HbA1c (ρ = 0.00) and FPG (ρ = 0.07), and overt and subclinical patterns had the same median HbA1c (6.6%). Thyroid function group alone explained 46% of HbA1c variance; continuous thyroid parameters, age, sex and body mass index explained 18%. Conclusions: The between-group differences and pooled correlations reflect a two-source design in which the comparison group was selected for normal glucose values. No graded relationship between thyroid function and glycemia was detectable within groups, and causal or directional inference is not supported.