Serum 25-hydroxyvitamin D and peripheral thyroid hormone sensitivity in euthyroid type 2 diabetes: Linear estimates and low-range threshold

Table 3 Subgroup analyses of the association between 25-hydroxyvitamin D and the FT3/FT4 ratio, with tests for interaction

Subgroup	n	β (95%CI)	P value	P for interaction
Age (year)				0.1131
< 60	812	0.04 (-0.03 to 0.12)	0.2645
≥ 60	459	0.13 (0.05-0.21)	0.0015
Sex				0.5338
Female	783	0.08 (0.01-0.14)	0.0223
Male	488	0.12 (0.03-0.21)	0.0087
Current smoking				0.3893
No	808	0.10 (0.04-0.17)	0.0010
Yes	375	0.05 (-0.05 to 0.15)	0.3672
Current drinking				0.0115
No	803	0.13 (0.07-0.19)	< 0.0001
Yes	379	-0.03 (-0.14 to 0.08)	0.6332
BMI (kg/m2)				0.6726
< 28	1020	0.09 (0.03-0.14)	0.0036
≥ 28	251	0.08 (-0.07 to 0.22)	0.3033

Estimates are from the fully adjusted linear model (model 3) within each stratum; β reflects the change in FT3/FT4 × 100 per 1 ng/mL higher 25-hydroxyvitamin D (25(OH)D). Subgroups were defined as age (< 60 years, ≥ 60 years), sex (women, men), current smoking (no, yes), current drinking (no, yes), and body mass index (< 28 kg/m², ≥ 28 kg/m²). P for interaction was obtained from the fully adjusted model in the overall sample by adding the product term (25(OH)D × subgroup), with main effects retained. Results are reported as β (95%CI) with two-sided P values; P < 0.05 indicate statistical significance. Analyses used a complete-case approach; stratum-specific n are shown and may vary due to missing covariates. No adjustment for multiple comparisons was applied. The outcome was modeled as FT3/FT4 × 100 for numerical scaling; inference is unaffected. BMI: Body mass index.