This clinical study aims to gain a better understanding of how different forms of vitamin D are processed in the body in healthy individuals, pregnant women, and patients with various hormonal (endocrine) and kidney (renal) disorders. In the long term, this study may provide new insights that could how vitamin D is tested and interpreted in these groups.

Vitamin D has several important roles in the body, such as building strong bones and maintaining calcium balance in the blood. Most vitamin D in the blood circulation is attached/bound to a protein called "vitamin D binding protein" (VDBP), which makes it unavailable for the body to use. A much smaller portion circulates freely in the blood and this is called "free vitamin D". This free form can be directly used by the body.

When your doctor tests your vitamin D levels, this usually refers to total vitamin D (the sum of bound and free vitamin D). However, this total value may not give an accurate indication of your actual vitamin D status, since most of it (the bound part) cannot be used by the body. The purpose of this study is to examine whether "free vitamin D" is a better marker of vitamin D status and if the amount of free vitamin D differs between healthy people, pregnant women, and people with specific endocrine or kidney disorders.

Additionally, this study will look into vitamin D metabolism more detailed, and investigate what different forms of vitamin D exist, how the body processes these, and whether these forms may be related to certain endocrine or kidney conditions.

1. General inclusion Criteria:

   - 18 years or above

2. General exclusion Criteria:

   • Below 18 years of age
   • Individuals incapable of providing informed consent

3. Cohort inclusion criteria

   • Obesity: BMI ≥ 25 kg/m2
   • Chronic Kidney Disease: ≥ CKD stage 3a (eGFR < 60 mL/min/1.73m2)
   • Pregnancy: in trimester 1 of pregnancy
   • Primary hyperparathyroidism: diagnosis at UZ Leuven and scheduled for parathyroidectomy at UZ Leuven
   • Complex calcium and phosphate disorders: diagnosis at UZ Leuven

4. Cohort exclusion criteria

   • Healthy controls: BMI below 18 kg/m² or above 25 kg/m², pregnancy, chronic kidney disease (independent of staging), primary hyperparathyroidism and vitamin D and/or calcium supplementation
   • Obesity: chronic kidney disease (independent of staging), pregnancy, primary hyperparathyroidism
   • Chronic Kidney Disease: BMI below 18 kg/m² or above 30 kg/m², pregnancy, primary hyperparathyroidism
   • Pregnancy: BMI below 18 kg/m² or above 30 kg/m², chronic kidney disease (independent of staging), primary hyperparathyroidism
   • Primary hyperparathyroidism: BMI below 18 kg/m² or above 30 kg/m², pregnancy, chronic kidney disease (independent of staging), secondary hyperparathyroidism

• Healthy controls: community sample
• Other cohorts: recruited from UZ Leuven (Academic medical center/tertiairy care center)