Kemik Homeostazında Modern Paradigma: PTH, D Vitamini ve FGF23

In this lecture, Prof. Dr. Kemal BUHARALIOĞLU explains the shift from an understanding of bone tissue as a passive structure to a perspective that views it as an active endocrine organ in light of recent developments. He provides up-to-date information on bone homeostasis through the delicate balance between parathyroid hormone (PTH), active vitamin D (calcitriol), and fibroblast growth factor (FGF) 23. This homeostatic system, an evolutionary legacy of the transition from oceans to land, explains the mechanisms by which medications like teriparatit (a PTH analogue) should be used intermittently (pulsatile) in pharmaceutical practice, why vitamin D supplements sometimes fail, and why calcium is an "essential" raw material for bone health. 1. PTH is an "orchestra conductor" that manages both bone formation and breakdown. PTH has coordinated effects in bone, kidney (increasing reabsorption by preventing calcium excretion in urine), and gut (increasing calcium absorption by converting vitamin D to its active form) to raise low calcium levels. Importance from a pharmaceutical practice perspective: It helps you explain to the patient that bone health is not just about "using calcium supplements," but is also an intertwined system with kidney and gut health. For example, in a patient using a proton pump inhibitor (PPI) that protects the stomach, calcium absorption from the intestines may be impaired due to reduced stomach acid. 2. The parathyroid glands are quite small and their anatomical location is strategic (thyroid Due to its visual similarities to bone tissue, patients who have had their thyroid gland removed are at risk of hypocalcemia. For patients discharged after thyroidectomy, the community pharmacist can be the first healthcare professional to notice potential complications. Importance from a pharmaceutical practice perspective: If a patient presenting to your pharmacy after thyroid removal surgery complains of tingling (paresthesia) and cramps around the hands and mouth, the patient should be referred to a specialist for urgent calcium supplementation. Serum calcium and PTH monitoring in the first few days after surgery and, if necessary, temporary oral calcium or calcitriol supplementation should be coordinated with the physician. 3. Although D3 (cholecalciferol) and D2 (ergocalciferol) undergo the same hydroxylation steps in the body, D3 is more potent clinically. 4. Activation of 1-alpha-hydroxylase in the kidneys and Vitamin D Synergy: PTH induces the 1-alpha-hydroxylase enzyme in the kidneys, leading to calcitriol formation. Calcitriol increases calcium absorption from the intestines, thus reducing calcium withdrawal from bone. It reduces the need. Therefore, PTH and vitamin D work synergistically. In a patient with vitamin D deficiency, PTH cannot utilize this 'logistical' support from the intestines, forcing it to draw calcium only from the bones. This inevitably exacerbates bone resorption. 5. The PTH-calcitriol-FGF23 trio manages the body's mineral traffic; it establishes calcium/phosphate balance through a feedback mechanism. While PTH increases calcium and tries to excrete phosphate through the kidneys, calcitriol increases the absorption of both from the intestines. FGF23, on the other hand, inhibits the 1-alpha-hydroxylase enzyme in the kidneys if phosphate levels rise excessively in the body (especially in kidney failure), thus stopping the activation of vitamin D and increasing phosphaturia. c) If you see the "High Phosphate + Low Calcium + High PTH" trio in a chronic kidney failure patient who comes to your pharmacy with biochemical analysis results, this indicates "secondary hyperparathyroidism," where bones are rapidly resorbed, and nephrology follow-up is vital for the patient. It should be explained. 6. Calcitriol increases calcium and phosphate absorption from the intestine; however, it can trigger bone resorption if dietary calcium is insufficient. 7. FGF23 and alpha-Klotho: FGF23 facilitates phosphate excretion in the kidney but cannot function without the Klotho protein. Importance from a Pharmaceutical Practice Perspective: It allows you to understand "vitamin D resistance" that develops due to decreased Klotho in aging or chronic kidney failure, and to interpret why standard vitamin D supplements are insufficient in these patients. 8. Vitamin D Brake: FGF23 is a "brake" system that stops vitamin D activation to prevent excessive phosphate accumulation. Importance from a Pharmaceutical Practice Perspective: It allows you to understand why high phosphate levels in kidney patients weaken bones; it explains why phosphate binders are critical for bone health in these patients. 9. Calcium/Phosphate Transport Dynamics in the Kidneys: PTH increases calcium reabsorption in the distal tubules, while It creates a phosphaturic effect by inhibiting sodium-phosphate cotransporters in the proximal tubules. This prevents the calcium-phosphate multiplication from exceeding the...