Week 11 - Calcium metabolism, Control of Metabolism

Question 1

Outline the steps involved in the activation of vitamin D.

Answer 1

1. Vitamin D is synthesised de novo in the skin or absorbed in the bowel as inactive D3. It undergoes two hydroxylation reactions to become active.
2. Firstly, in the liver to 25-OH-D3, and then in the kidney to 1,25-(OH)2-D3. The second step is catalysed by 1-alpha-hydroxylase and is the rate-limiting step.
3. Active vitamin D regulates bone turnover, and increases calcium reabsorption in the kidney and bowel.

Question 2

What is the most common cause of primary hyperparathyroidism?

Answer 2

Usually occurs due to an intrinsic pathology of the parathyroid gland, which is most often a benign parathyroid adenoma
Hyperplasia and malignant tumours are far more rare

Question 3

Describe the effects of parathyroid hormone.

Answer 3

1. Acts on bone to stimulate resorption which releases calcium and phosphate (it increases the activity of osteoclasts)
2. In the kidney, PTH increases reabsorption of calcium and promotes phosphate excretion. It increases synthesis of 1-alpha hydroxylase, the enzyme needed for activation of vitamin D.