Parathyroid hormone, calcium and bone Flashcards
Functions of the skeleton
- mechanical; provides support and muscle attachments
- protective; for organs and bone marrow
- metabolic; ion homeostasis, especially calcium and phosphate
Structure of bone
Bone is made of specialised connective tissue. The extracellular matrix is able to calcify via mineralisation, which occurs with formation of hydroxyapatite crystals. The fibres are collagen (90%) and non-collagenous proteins (osteocalcin, osteonectin, osteopontin). Several types of cells are present.
Types of bone
- compact (cortical) bone; a hard outer layer that is dense, strong and durable
- cancellous (trabecular/spongy) bone; lighter, less dense and more flexible, consisting of a network of trabeculae
Cell types in bone
- osteocytes; embedded in the calcified bone matrix, have long processes
- osteoblasts; bone forming cells
- osteoclasts; bone resorbing cells
Osteoblasts
Bone forming cells, which produce matrix components and aid calcification. These originate from mesenchymal stem cells. Markers are alkaline phosphatase and osteocalcin.
Osteoclasts
Bone resorbing cells, which produce acid to resorb mineral and enzymes to resorb the matrix. They eat away at bone from the surface inwards, at resorption pits/lacunae. These cells originate from the bone marrow lineage, so are multinucleate. They bind to bone via integrins. Markers are carbonic anhydrase, tartrate-resistant acid phosphatase (TRAP), RANK and the calcitonin receptor.
Osteoclast activation
Osteoblasts can activate osteoclast precursor cells via the RANK ligand/receptor interaction. The expression of the RANK ligand by osteoblasts is regulated by factors such as vitamin D, parathyroid hormone and prostaglandins.
Osteoclast inhibition
A decoy receptor called osteoprotegrin (OPG) dampens the activation of osteoclasts by binding to the RANK ligand. It’s expression is regulated by factors such as oestrogens, IL-4 and TGFb.
Denosumab is an anti-osteoporotic drug which acts as a monoclonal antibody for the RANK ligand.
Bone remodeling
Bone is a dynamic living tissue, constantly being resorbed and formed via the activity of osteoblasts and osteoclasts. In adults, the entire skeleton is replaced every ~10 years, and peak bone mass is reached around 20-30y. Remodeling allows the body to fix damaged sections, reshape the skeleton during growth, and regulate calcium levels. It is under the control of several hormones, including parathyroid hormone, calcitonin, vitamin D, estrogen in women, and testosterone in men.
Vitamin D
A fat soluble steroid with hormonal activity. It is important for bone development and metabolism and works together with parathyroid hormone (PTH) to maintain bone health.
Functions of vitamin D
- calcium absorption in bowel
- maintaining serum calcium and phosphate levels
- osteoid mineralisation and bone remodelling
- neuromuscular function
- immune function
- reduction of inflammation
Sources of vitamin D
Vitamin D3 (calciol, cholecalciferol) is synthesised from cholesterol in the skin upon exposure to UV, and is also found in some animal products/fortified foods eg. oily fish, eggs, milk, cereals.
Vitamin D2 (ergocalciferol) is found in some plant foods, eg. mushrooms.
Physiology of vitamin D
In the liver, vitamin D is converted by 25-hydroxylase into 25-hydroxy-vitamin D (calcidiol), which is released into the blood and binds to vitamin D binding proteins. This is the major storage form of vitamin D, mainly produced during the summer.
In the kidneys, 1-alpha hydroxylase produces 1,25-dihydroxy-vitamin D (calcitriol) in response to low blood calcium or phosphate levels. This is the biologically active form of vitamin D.
Risk factors for vitamin D deficiency
- living in northern latitude
- excessive suncream use/covering clothing
- air pollution (reduced UVB)
- obesity
- pregnancy, breastfeeding and infancy
- being older/housebound
- bowel disease (absorption issues)
- dark skin pigmentation
Presentation of vitamin D deficiency
Mostly asymptomatic, presents if deficiency is prolonged:
- tingling numbness + very late tetany
- muscle pain + proximal weakness
- seizures
- hypocalcaemic dilated cardiomyopathy (infants)
- osteomalacia: fractures, back and pelvis pain
- rickets (children): soft, pliable bones
Investigations for vitamin D deficiency
The first line investigation for vitamin D deficiency is serum 25-hydroxy-vitamin D, or 1,25-dihydroxy-vitamin D levels in patients with renal disease.
- <25-30nmol/L = deficiency
- > 50nmol/L = sufficiency
Treatment for vitamin D deficiency
Vitamin D replacement is considered if serum 25(OH)D levels are <25nmol/L, or 25-50nmol/L if other criteria are met. Treatment with vitamin D3 is preferred, but vitamin D2 is also suitable. A high loading dose is used to quickly correct the deficiency, then a lower maintenance dose is given.
Osteoporosis
A metabolic bone disease secondary to an imbalance between bone breakdown and remodelling (osteoclast and osteoblast activity). Reduced bone mass and structural deterioration leads to increased bone fragility and susceptibility to fracture.
Risk factors for primary osteoporosis
- old age
- prolonged immobility
- menopause
Risk factors for secondary osteoporosis
- low BMI
- steroid use
- tobacco and alcohol
- endocrine conditions (eg. diabetes, Cushing’s disease, hypogonadism, hyperthyroidism, hyperparathyroidism)
- liver or renal failure
- dietary deficiencies (eg. calcium, protein)
Investigation of osteoporosis
Osteoporosis is asymptomatic, and generally not found until injury occurs. Assessment of osteoporosis includes calculating 10y fracture risk. If intermediate or high (=/>10%), a dual-energy X-ray absorptiometry (DXA) scan can be used to measure bone mineral density (BMD).
Prevention of osteoporosis
If fracture risk is low/intermediate, prevention may be with lifestyle changes, such as a nutrient-rich diet, exercise, reduction of alcohol and tobacco, and maintaining a healthy BMI.
Parathyroid hormone (PTH)
PTH is released by the parathyroid glands in response to low calcium levels. It has 3 main actions:
- increasing osteoclast activity (bone resorption) and increasing Ca and PO4 release from bones
- increasing Ca and decreasing PO4 absorption in the kidneys
- increasing active vitamin D (1,25(OH)2D) by activating 1-alpha-hydroxylase in the kidneys
Types of hyperparathyroidism
Primary: the parathyroid gland produces excess PTH, independently of calcium levels, due to adenoma or hyperplasia.
Secondary: hypocalcaemia causes excess PTH to be released. Commonly caused by chronic kidney disease or vitamin D deficiency.
Tertiary: prolonged secondary hyperparathyroidism results in the glands acting autonomously, leading to hypercalcaemia.
Presentation of hyperparathyroidism
Most cases are asymptomatic and are found incidentally. Symptoms are usually a manifestation of hypercalcaemia:
- STONES: renal calculi, raised GFR, polyuria, nocturia, thirst
- BONES: pain from demineralisation, low BMD
- GROANS: nausea and vomiting, constipation, indigestion
- MOANS: lethargy, fatigue, confusion, depression
Investigation of hyperparathyroidism
Biochemical analysis is used to diagnose hyperparathyroidism:
- elevated PTH is found in all types.
- primary/tertiary; high Ca, low/normal PO4, low vitamin D
- secondary; low Ca, low/normal PO4, low vitamin D
Treatment of hyperparathyroidism
Primary: parathyroidectomy (98-99% cure rate, complications include hypoparathyroidism and recurrent laryngeal nerve damage).
Secondary: vitamin D supplementation in deficiency, 1,25-dihydroxy-vitamin D and phosphate binders in renal failure
Tertiary: surgery/calcimimetics
