Table 3. Linear regression model of factors associated with serum 25-hydroxyvitamin D (nmol/L), 1,25-dihydroxyvitamin D (pmol/L) and parathyroid hormone concentrations (ng/L) among Thai schoolchildren aged 6–13 y¹.

	Unadjusted				Adjusted/final2 , 3
Variable	n	Estimates	95% CI	P	n	Estimates	95% CI	P
For 25(OH)D					525
BMI	525	0.99	0.98, 1.00	0.021		0.997	0.987, 1.01	0.515
Sex (male)	526	1.19	1.14, 1.23	<0.001		1.19 4	1.14, 1.23	<0.001
Age, y	526	0.98	0.96, 0.99	0.010
Age, y for boys						1.00	0.98, 1.02	0.895
Age, y for girls						0.96	0.94, 0.97	0.001
Sex × Age interaction								<0.001
Height-for-age Z score	526	0.97	0.94, 0.99	0.017		0.98	0.96, 0.99	0.006
For 1,25(OH)2D					510
BMI	518	1.03	1.00, 1.06	0.028		1.11	1.04, 1.18	0.004
BMI-squared		0.996	0.994, 0.999	0.009		0.997	0.996, 0.999	0.004
Sex (male)	519	0.86	0.83, 0.89	<0.001		0.86	0.82, 0.90	<0.001
Age, y	519	1.02	0.99, 1.05	0.232		1.02	0.99, 1.04	0.17
Height-for-age Z score	519	1.07	1.05, 1.110	<0.001		1.07	1.04, 1.11	0.001
log 25OHD	512	0.91	0.76, 1.08	0.235		1.12	0.95, 1.33	0.153
For PTH					502
BMI	514	1.02	0.99, 1.04	0.196		0.98	0.96, 1.02	0.365
Sex (male)	515	0.80	0.74, 0.87	<0.001		0.91	0.82, 1.02	0.097
Age, y	515	1.04	1.01, 1.07	0.008		1.03	0.999, 1.07	0.055
Age-squared		1.02	1.00, 1.04	0.037		1.02	1.00, 1.03	0.049
Height-for-age Z score	515	1.10	1.03, 1.18	0.012		1.08	1.02, 1.14	0.016
log 25OHD	509	0.50	0.43, 0.57	<0.001		0.61	0.49, 0.74	<0.001
log 1,25(OH)2D	509	1.34	1.09, 1.64	0.012		1.18	0.99, 1.40	0.06

¹Effect sizes are ratios of geometric means, accompanied by 95% CIs and P-values.

²Adjusted for variables with P≤0.25 in univariate analysis.

³R² of the whole model for 25OHD: 0.24; for 1,25(OH)₂D: 0.13; for PTH: 0.20.

⁴At the mean age (9.9 y).