What is Phosphate Diabetes?

Phosphate diabetes is a condition characterized by excessive phosphate loss through the urine due to reduced phosphate reabsorption in the kidneys. This leads to low phosphate levels in the blood (hypophosphatemia) without the expected decrease in urinary phosphate excretion.

Why is it called “Diabetes”?

Phosphate diabetes is termed "diabetes" because it involves an abnormal loss of a substance (in this case, phosphate) through the urine, similar to how diabetes mellitus involves excessive glucose loss through urine. The word "diabetes" originates from a Greek word meaning "to pass through," referring to the excessive urination seen in these conditions.

In phosphate diabetes, the kidneys fail to properly reabsorb phosphate, leading to its excessive excretion in the urine. This parallels how, in diabetes mellitus, the body fails to reabsorb glucose effectively, resulting in its loss through urine. Therefore, the term "diabetes" is used to highlight this abnormal loss of essential substances in both conditions.

Diagnosis of Phosphate Diabetes

Diagnosing phosphate diabetes involves measuring phosphate clearance, the maximal rate of tubular phosphate reabsorption (TmPO4), and the ratio of TmPO4 to the glomerular filtration rate (TmPO4/GFR). Here’s how each of these factors is assessed:

• Phosphate Clearance: Calculated by multiplying urinary phosphate excretion by urine output and then dividing by plasma phosphate levels. Phosphate clearance above 15 ml/min suggests phosphate diabetes.
• TmPO4/GFR Ratio: A TmPO4/GFR below 0.8 is a strong indicator of phosphate diabetes.

Consequences of Phosphate Diabetes

• Children: Vitamin D-resistant rickets.
• Adults: Osteomalacia, osteoporosis, joint pain, muscle weakness, and occasionally kidney stones.

Causes of Phosphate Diabetes:

1. Phosphate Diabetes Associated with Tumors

   • Commonly linked to benign tumors like hemangiopericytomas, characterized by slow growth. Treatment often involves surgery, with a high recovery rate.

2. Inherited Phosphate Diabetes

   • X-linked Hypophosphatemic Rickets (XLH): Causes growth failure and other skeletal issues in children.
   • Autosomal Dominant Hypophosphatemic Rickets (ADHR): Leads to bone pain and deformities.

3. Phosphate Diabetes from Complex Kidney Disorders

   • Includes conditions like Fanconi syndrome, where phosphate, glucose, and amino acids are lost through urine.

4. Primary Hyperparathyroidism

   • Overactive parathyroid glands can lead to phosphate diabetes.

5. Moderate Idiopathic Phosphate Diabetes in Adults

   • Often seen in everyday rheumatology, presenting as fibromyalgia, chronic back pain, or osteoporosis. The exact cause is unknown.

Reference:

• Laroche M, Boyer JF. Phosphate diabetes, tubular phosphate reabsorption and phosphatonins. Joint Bone Spine. 2005 Oct 1;72(5):376-81.