• Causes of this condition include a vitamin D-deficient diet, inadequate sunlight exposure, poor absorption of vitamin D, and enzyme deficiencies in the pathway to vitamin D activation.
• In all scenarios, there is a decrease in plasma calcium, which triggers an increase in PTH secretion and a condition known as secondary hyperparathyroidism.
• This condition is characterized by increased PTH, decreased plasma calcium, and decreased plasma phosphate. Although the elevated PTH enhances phosphate resorption from bone, it also inhibits phosphate reabsorption by the kidney, thereby promoting phosphate excretion and a decrease in plasma phosphate levels.
• A decrease in vitamin D levels can cause rickets in children and osteomalacia in adults.
• To maintain plasma calcium levels, bone mass is lost.

Note that:

In patients with celiac disease, the small intestinal mucosa becomes inflamed and atrophic in response to gluten, leading to impaired mucosal absorption of vitamins and nutrients.

Malabsorption of vitamin D occurs due to excess excretion in bulky, fatty stools. The resulting decrease in vitamin D levels causes rickets in children and osteomalacia in adults.

• In the case of vitamin D deficiency, the absorption of both calcium and phosphorus from the gastrointestinal tract is significantly reduced. The low levels of 1,25-dihydroxy vitamin D and the resulting hypocalcemia lead to an increase in parathyroid hormone (PTH) levels, which is known as secondary hyperparathyroidism.
• This increase in PTH leads to the release of calcium and phosphorus from the bones, reducing bone mineralization and potentially causing symptoms of osteomalacia.
• Although PTH works to increase serum calcium concentrations, these levels often remain low in states of vitamin D deficiency, especially when bone stores are depleted, as seen in this osteomalacic patient.
• Vitamin D deficiency also results in a decrease in serum phosphorus due to poor gastrointestinal phosphate absorption and increased urinary phosphate excretion induced by high PTH levels.

Rickets and Osteomalacia:

• These conditions originate from the abnormal mineralization of bone and cartilage.
• Rickets occurs before plate closure, while osteomalacia occurs after plate closure.
• Rickets is characterized by expansion of the epiphyseal plates with the most noticeable abnormalities being leg bowing and a protuberant abdomen.
• Osteomalacia symptoms are more subtle.

Osteoporosis:

• Osteoporosis involves a loss of bone mass leading to fractures. This condition is due to normal age-related changes in bone remodeling, along with other factors that exacerbate this process.
• A key part of treatment involves the use of bisphosphonates, which are quickly incorporated into bone and reduce the activity of osteoclasts.