What is the 1,25-Dihydroxyvitamin D (Calcitriol) Test?

The 1,25-dihydroxyvitamin D test measures calcitriol, the hormonally active form of vitamin D responsible for regulating calcium and phosphate balance in the body. Unlike the more commonly tested 25-hydroxyvitamin D, calcitriol reflects vitamin D activation rather than overall vitamin D stores.

Calcitriol is produced through a two-step process: vitamin D is first converted in the liver, then activated in the kidneys. This active form directly influences calcium absorption from the gut, bone metabolism and parathyroid hormone (PTH) activity.

Because calcitriol levels are tightly regulated, this test is used in specific clinical situations rather than routine vitamin D screening.

Why do I need a test for 1,25-Dihydroxyvitamin D?

You may need this test if there are abnormalities in calcium, phosphate or parathyroid hormone levels that cannot be explained by standard vitamin D testing. Calcitriol measurement is particularly important when kidney function is impaired, as the kidneys play a key role in vitamin D activation.

This test is also used when certain inflammatory or granulomatous diseases are suspected, as these conditions can cause excessive conversion of vitamin D to its active form, leading to high calcium levels.

Symptoms and scenarios where testing is recommended:

• Unexplained high or low calcium levels
• Chronic kidney disease
• Abnormal parathyroid hormone (PTH) results
• Suspected vitamin D metabolism disorders
• Sarcoidosis or other granulomatous disease
• Unexplained bone pain or fractures
• Disorders of phosphate balance
• Investigation of calcium-related kidney stones

Benefits of the 1,25-Dihydroxyvitamin D Test

• Assesses active vitamin D function
  Measures the hormone responsible for calcium and phosphate regulation.
• Clarifies complex calcium disorders
  Helps explain unexplained hypercalcaemia or hypocalcaemia.
• Essential in kidney disease assessment
  Identifies impaired vitamin D activation in renal dysfunction.
• Supports parathyroid disorder diagnosis
  Interpreted alongside PTH to assess calcium-hormone balance.
• Identifies abnormal vitamin D activation
  Useful in granulomatous and inflammatory diseases.
• Prevents misinterpretation of vitamin D status
  Differentiates storage deficiency from activation problems.
• Guides specialist endocrine or renal care
  Results influence further investigation and treatment.
• Targeted, clinically specific testing
  Used when standard vitamin D tests are insufficient.

Step-by-Step: How Testing Works

Choose your preferred testing method

You can attend one of our UK partner clinics or arrange a home or hotel visit with a mobile phlebotomist.

Option 1 – In-Clinic Blood Draw

1. Select your nearest UK clinic and book an appointment.
2. A clinician or phlebotomist collects a venous blood sample.
3. The sample is analysed for calcitriol levels.
4. Results are delivered securely online.

Option 2 – Home or Hotel Phlebotomy (+£60)

1. Book a UK-wide home or hotel visit at a convenient time.
2. A trained phlebotomist collects your blood sample on-site.
3. The sample is packaged and transported to the laboratory.
4. Results are uploaded digitally without the need to travel.

Access your report

You can download and share your results with your GP, endocrinologist or renal specialist.

Your Results Explained

Low Levels

Low calcitriol levels suggest reduced vitamin D activation, most commonly due to kidney disease or severe vitamin D deficiency.
This can lead to poor calcium absorption and bone mineral loss.
Typical UK reference range: approximately 48–190 pmol/L (ranges vary by laboratory).

Normal Levels

Normal results indicate appropriate vitamin D activation and balanced calcium regulation.
This suggests the kidneys and parathyroid system are functioning as expected.

High Levels

Elevated calcitriol levels indicate excessive vitamin D activation.
This may occur in granulomatous diseases, certain lymphomas or rare metabolic disorders and is often associated with high calcium levels.
Further investigation is usually required to identify the underlying cause.