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

Table 1 Baseline characteristics across quartiles of the FT3/FT4 ratio

Characteristic	Q1 (n = 331)	Q2 (n = 373)	Q3 (n = 347)	Q4 (n = 357)	P value
Age (years)	55.43 ± 13.13	55.07 ± 13.27	53.21 ± 12.87	51.23 ± 12.39	< 0.001
Sex					< 0.001
Female	143 (43.20)	171 (45.84)	99 (28.53)	110 (30.81)
Male	188 (56.80)	202 (54.16)	248 (71.47)	247 (69.19)
Current drinking					0.005
No	224 (72.0)	245 (71.6)	204 (62.6)	207 (62.3)
Yes	87 (28.0)	97 (28.4)	122 (37.4)	125 (37.7)
Current smoking					0.002
No	230 (74.0)	246 (71.5)	204 (62.6)	211 (63.6)
Yes	81 (26.0)	98 (28.5)	122 (37.4)	121 (36.4)
DBP (mmHg)	74.72 ± 9.31	74.48 ± 9.08	75.80 ± 8.55	76.39 ± 9.11	0.037
SBP (mmHg)	125.68 ± 14.30	127.56 ± 14.43	127.34 ± 13.2	126.71 ± 12.78	0.289
HbA1C	10.47 (8.38-12.28)	9.62 (7.56-11.14)	8.90 (7.34-10.67)	8.32 (7.11-9.80)	< 0.001
TC (mmol/L)	4.49 (3.81-5.47)	4.62 (3.96-5.44)	4.69 (4.04-5.35)	4.67 (3.79-5.51)	0.633
HDL-C (mmol/L)	1.12 (0.93-1.29)	1.10 (0.94-1.33)	1.08 (0.87-1.28)	1.08 (0.93-1.27)	0.245
LDL-C (mmol/L)	2.80 ± 1.05	2.79 ± 1.00	2.81 ± 1.00	2.73 ± 0.99	0.768
TG (mg/dL)	1.61 (1.08-2.66)	1.76 (1.20-2.95)	2.16 (1.38-3.36)	2.05 (1.30-3.60)	< 0.001
eGFR (mL/minute/1.73 m2)	102.12 ± 15.40	102.26 ± 16.26	102.78 ± 14.36	106.89 ± 14.40	< 0.001
UA (μmol/L)	308.60 (253.57-369.17)	305.20 (245.80-371.20)	319.05 (277.42-381.00)	326.60 (274.77-392.47)	0.002
AST(U/L)	17.40 (14.10-24.70)	18.30 (15.22-24.70)	18.70 (15.20-25.42)	21.05 (16.12-29.05)	< 0.001
ALT(U/L)	19.20 (13.30-30.80)	21.70 (15.57-36.60)	23.20 (15.20-37.80)	27.10 (18.25-45.85)	< 0.001
CRP (mg/L)	2.04 (0.99-4.58)	1.56 (0.69-3.70)	1.50 (0.78-3.25)	1.44 (0.71-3.23)	< 0.001
WBC (× 109/L)	6.49 ± 2.14	6.31 ± 1.73	6.44 ± 1.78	6.58 ± 1.91	0.519
WC (cm)	86.60 (81.50-94.00)	89.00 (82.00-95.00)	89.00 (84.00-96.00)	89.10 (84.60-95.50)	< 0.001
VFA (cm2)	86.00 (63.00-110.00)	93.00 (70.00-114.00)	96.00 (72.00-123.00)	99.00 (75.75-122.25)	< 0.001
SFA (cm2)	162.00 (123.00-204.00)	162.50 (126.00-208.25)	168.00 (135.00-220.00)	169.50 (133.75-216.75)	0.038
FT3 (pmol/L)	3.43 ± 0.40	3.84 ± 0.36	4.10 ± 0.38	4.43 ± 0.45	< 0.001
FT4 (pmol/L)	13.83 ± 1.42	12.94 ± 1.18	12.41 ± 1.13	11.65 ± 1.13	< 0.001
TSH (μIU/mL)	1.25 (0.83-1.86)	1.43 (0.99-2.07)	1.41 (1.00-1.99)	1.46 (0.99-2.12)	0.004
BMI (kg/m2)	24.51 ± 3.74	25.12 ± 3.51	25.60 ± 3.58	26.21 ± 4.30	< 0.001
25(OH)D (ng/mL)	22.14 ± 9.73	22.28 ± 7.40	22.68 ± 7.99	23.38 ± 7.70	0.105

Continuous variables are presented as mean ± SD if approximately normal or median (interquartile range) if skewed; categorical variables as n (%). Quartiles (Q1-Q4, from lowest to highest) were defined by the FT3/FT4 ratio. Between-group differences were tested with one-way ANOVA or Kruskal-Wallis tests for continuous variables and χ² tests for categorical variables, as appropriate. Two-sided P < 0.05 was considered statistically significant. All comparisons are descriptive, reflecting distributional differences across quartiles; sample counts may vary by variable due to missing data. No adjustment for multiple comparisons was applied. DBP: Diastolic blood pressure; SBP: Systolic blood pressure; HbA1C: Glycated hemoglobin; TC: Total cholesterol; HDL-C: High-density lipoprotein cholesterol; LDL-C: Low-density lipoprotein cholesterol; TG: Triglycerides; eGFR: Estimated glomerular filtration rate; UA: Uric acid; AST: Aspartate aminotransferase; ALT: Alanine aminotransferase; CRP: C-reactive protein; WBC: White blood cell count; WC: Waist circumference; VFA: Visceral fat area; SFA: Subcutaneous fat area; TSH: Thyroid-stimulating hormone; BMI: Body mass index; 25(OH)D: 25-hydroxyvitamin D.

Table 2 Association between 25-hydroxyvitamin D and the FT3/FT4 ratio in multivariable linear models

Exposure	Crude model (model 1), n = 1271		Partially adjusted model (model 2), n = 1271		Fully adjusted model (model 3), n = 577
	β (95%CI)	P value	β (95%CI)	P value	β (95%CI)	P value
25(OH)D	0.045 (0.011-0.079)	0.01043	0.058 (0.022-0.093)	0.00163	0.088 (0.037-0.139)	0.00081
25(OH)D quartile
Q1	Reference		Reference		Reference
Q2	0.894 (0.099-1.690)	0.02776	1.043 (0.253-1.833)	0.00980	0.846 (-0.251 to 1.943)	0.13103
Q3	1.085 (0.290-1.880)	0.00756	1.270 (0.468-2.072)	0.00195	1.517 (0.362-2.671)	0.01029
Q4	0.865 (0.070-1.660)	0.03317	1.129 (0.298-1.959)	0.00785	1.246 (0.074-2.419)	0.03772
P for trend	0.03014		0.00789		0.02381

Outcome: FT3/FT4 × 100 (continuous). Exposure: 25-hydroxyvitamin D [25(OH)D; continuous; β reflects the change in FT3/FT4 × 100 per 1 ng/mL higher 25(OH)D], with additional analyses by quartiles (Q1-Q4; Q1 as reference) and P for trend. Trend tests were obtained by entering the median of each quartile as a continuous term. Model 1: Unadjusted; model 2: Adjusted for age, sex; model 3: Additionally adjusted for body mass index, systolic blood pressure, diastolic blood pressure, glycated hemoglobin (Box-Cox), total cholesterol (Box-Cox), high-density lipoprotein cholesterol (Box-Cox), low-density lipoprotein cholesterol, triglycerides (Box-Cox), estimated glomerular filtration rate (Box-Cox), uric acid (Box-Cox), aspartate aminotransferase (Box-Cox), alanine aminotransferase (Box-Cox), C-reactive protein (Box-Cox), white blood cell count (Box-Cox), visceral fat area (Box-Cox), subcutaneous fat area (Box-Cox), current smoking, and current drinking. Estimates are β (95%CI) with two-sided P values; P < 0.05 was considered statistically significant; bold P values indicate significance. The outcome was modeled as FT3/FT4 × 100 for numerical scaling; inference is unaffected. 25(OH)D: 25-hydroxyvitamin D.

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.

Table 4 Threshold effect of 25-hydroxyvitamin D on the FT3/FT4 ratio from two-piecewise linear regression

FT3/FT4	Adjusted β (95%CI)	P value
25(OH)D (model I)	0.088 (0.036-0.140)	0.0010
25(OH)D (model II)
Inflection point	14.7 (12.85-16.11)
25(OH)D < 14.7	0.61 (0.31-0.92)	< 0.0001
25(OH)D ≥ 14.7	0.04 (-0.02 to 0.10)	0.2048
Difference (segment 2-1)	-0.57 (-0.91 to -0.24)	0.0007
Log likelihood ratio		< 0.001

Two-piecewise (segmented) linear regression with an estimated inflection at 14.7 ng/mL (95%CI: 12.85-16.11) of 25-hydroxyvitamin D [25(OH)D]. Slopes (β) represent the change in FT3/FT4 × 100 per 1 ng/mL higher 25(OH)D below and at/above the inflection, reported as β (95%CI) with two-sided P values. “Difference (segment 2-1)” is the contrast between the slope at/above vs below the inflection. “Model I” denotes the single-line linear model; “model II” the two-piecewise model. Model comparison vs the single-line specification used the likelihood-ratio test. The inflection point and its 95%CI were obtained by bootstrap resampling. Both model I and model II were fitted in the same complete-case sample (n = 577). All models were fully adjusted for age, sex, body mass index, systolic blood pressure, diastolic blood pressure, glycated hemoglobin (Box-Cox), total cholesterol (Box-Cox), high-density lipoprotein cholesterol (Box-Cox), low-density lipoprotein cholesterol, triglycerides (Box-Cox), estimated glomerular filtration rate (Box-Cox), uric acid (Box-Cox), aspartate aminotransferase (Box-Cox), alanine aminotransferase (Box-Cox), C-reactive protein (Box-Cox), white blood cell count (Box-Cox), visceral fat area (Box-Cox), subcutaneous fat area (Box-Cox), current smoking, and current drinking. No adjustment for multiple comparisons was applied. The outcome was modeled as FT3/FT4 × 100 for numerical scaling; inference is unaffected. 25(OH)D: 25-hydroxyvitamin D.