Prevalence and determinants of secondary hyperparathyroidism: IMOS-2021.

Secondary hyperparathyroidism (SHPT) is a common endocrine disorder among older adults, linked to adverse bone and cardiovascular outcomes. Despite its clinical significance, population-based data on SHPT prevalence and determinants, especially in Middle Eastern thancountries, including Iran, remain scarce. Identifying modifiable and non-modifiable risk factors is critical for targeted prevention and management strategies. This cross-sectional study analyzed data from the Iranian Multicenter Osteoporosis Study (IMOS-2021), a nationally representative cohort of adults aged ≥ 50 years. The weighted prevalence of SHPT was estimated in the 1369 participants. After excluding individuals with possible primary hyperparathyroidism, low PTH, and unknown elevated PTH, participants with SHPT were compared to those with normal PTH regarding demographic, biochemical, and clinical characteristics. Multivariable logistic regression identified independent determinants of SHPT, while linear regression assessed factors influencing serum parathyroid hormone (PTH) levels. The prevalence of SHPT was 12.34% (10.49-14.46). Compared to normal participants (n = 1161), individuals with SHPT (n = 174) had higher body mass index (29.7 vs. 28.0 kg/m2), lower serum vitamin D (35 vs. 46 ng/mL), and reduced renal function (eGFR: 81 vs. 85 mL/min/1.73 m2). Higher serum vitamin D levels were negatively associated with SHPT: adjusted odds ratios (aOR) were 0.3 (0.16-0.56) for vitamin D 30-50 ng/mL, 0.16 (0.08-0.34) for 50-70 ng/mL, and 0.31 (0.15-0.64) for ≥ 70 ng/mL, relative to < 20 ng/mL. Age ≥ 65 years (1.7; 1.15-2.51), obesity (aOR = 1.92; 1.15-3.18), low calcium intake (aOR = 1.82; 1.2-2.77), and reduced renal function (aORs of 1.79 (1.18-2.72) and 2.21 (1.09-4.48), for eGFR 60-90 and < 60 mL/min/1.73 m2, respectively) were significantly associated with SHPT. In linear models, vitamin D > 30 ng/mL correlated with lower PTH levels (adjusted β ranging from - 5.8 to -10.7 for the 30-50, 50-70, and > 70 ng/mL categories relative to < 20 ng/mL. Conversely, age ≥ 65 years (β = 3.73; 1.3 to 6.16), BMI ≥ 30 kg/m2 (β = 3.32; 0.72 to 5.93), low calcium intake (β = 2.4; 0.38 to 4.42), eGFR < 60 mL/min/1.73 m2 (β = 6.38; 2.57 to 10.18) and 60-90 (β = 3.62; 1.58 to 5.67) were associated with higher PTH levels. Additionally, phosphorus level (β = -2.36; -4.38 to -0.34), and male sex (β = -4.54; -6.77 to -2.31) was independently associated with lower PTH concentrations. This study provides nationally representative evidence on SHPT prevalence and its determinants in Iranian adults. Obesity, low calcium intake, and chronic kidney disease were positively associated with SHPT and PTH elevation, while Vitamin D sufficiency, phosphorus level, and male sex were negatively associated with the condition. These findings underscore the importance of screening and addressing modifiable factors to reduce SHPT in aging populations.