parathyroid hormone Flashcards
What form is calcium stored as in bone?
Calcium carbonate, in the hydroxyapatite matrix
What is the role of parathyroid hormone?
Regulates calcium and phosphate homeostasis
What are the 3 main targets of PTH?
Bone, Kidneys, Intestines
What is the action of PTH on bone?
Stimulates osteoclast activity, to break down bone and release stored calcium and phosphate into the blood stream
What is the action of PTH on the kidneys?
Reduces phosphate reabsorption/increases urinary excretion, increases calcium reabsorption.
Also stimulates calcitriol production.
What is the action of PTH on the intestines?
Calcitriol released in response to PTH activity in the kidneys enhances gut calcium and phosphate absorption
What is the net effect of PTH on phosphate levels?
Net decrease in phosphate - although bone breakdown and gut absorption increase levels, the increased renal clearance has a greater effect and lowers phosphate
What are the negative feedback loops controlling PTH secretion?
High calcium levels due to PTH action on bone/kidneys/intestines provides negative feedback to the parathyroid gland
What is the function of calcitonin?
Function is poorly understood in mammals, but role is generally to decrease calcium levels by reducing bone turnover and increasing renal excretion
How does calcium travel in the blood?
Bound to albumin
Why is it important to measure albumin levels in a patient with calcium dysfunction?
Calcium in the body is bound to albumin. A low albumin will make the calcium appear lower than it really is and vice versa.
What is primary hyperparathyroidism?
Excessive, uncontrolled secretion of PTH from the parathyroid glands, usually due to an adenoma/carcinoma
What is secondary hyperparathyroidism?
Low calcium levels from CKD, vitamin D deficiency or malabsorption syndromes stimulate an excessive PTH production.
What is tertiary hyperparathyroidism?
Long standing secondary hyperparathyroidism and low calcium causes the parathyroid galnds to become autonomous and overactive, so they continue to secrete excess PTH, even once calcium levels are corrected.
How does chronic kidney disease cause secondary hyperparathyroidism?
The damaged kidneys are unable to activate calcitriol, so there will be reduced calcium uptake from the gut. Renal phosphate excretion will be impaired so levels will be high. Together, this will trigger release of PTH, which will be inadequate to correct calcium due to the damaged kidneys
What is the pathophysiology of secondary hyperparathyroidism caused by vitamin D/calcium deficiency?
Low levels of calcium due to gut absorption or inadequate vitamin D activation will increase PTH levels to compensate.
What are the symptoms of hyperparathyroidism?
Bone pain, bone fractures, muscle weakness, fatigue, cystic bone lesions, kidney stones, polyuria, constipation, cognitive disturbances, arrhythmias
What are the relative phosphate and calcium levels in primary hyperparathyroidism?
Calcium will be high, phosphate will be low
What are the relative phosphate and calcium levels in secondary hyperparathyroidism caused by CKD?
Calcium will be decreased, phosphate will be increased
What are the relative phosphate and calcium levels in secondary hyperparathyroidism caused by Ca/Vit D deficiency?
Decreased calcium, decreased phosphate
What are the relative phosphate and calcium levels in tertiary hyperparathyroidism?
Increased calcium, decreased phosphate
What are the results of a 24 hour urinary calcium excretion test in the different types of hyperparathyroidism?
Calcium excretion is elevated in primary and tertiary hyperparathyroidism, low/normal in secondary hyperparathyroidism
What are some of the investigations that can be used to diagnose hyperparathyroidism?
Bloods testing calcium, phosphate, albumin, PTH. Bone density scan may show osteoporosis or osteopenia, ultrasound may show parathyroid tumo
What are the treatment options for primary hyperparathyroidism?
Removal of tumour, surgical parathyroidectomy
What are the treatment options for secondary hyperparathyroidism?
Vitamin D/calcitriol supplements
What are the treatment options for tertiary hyperparathyroidism?
Complete parathyroidectomy
What are the causes of hypoparathyroidism?
Thyroidectomy/parathyroidectomy, autoimmunity, congenital abnormality of parathyroid glands, pseudo-hypo-parathyroidism (not technically hypoparathyroidism)
What are the symptoms of hypoparathyroidism?
muscle cramps/spasms, paraesthesia, irritability, anxiety, seizures, prolonged QT interval, hypotension, osteosclerosis
What are the relative calcium and phosphate levels in hypoparathyroidism?
low calcium, high phosphate
What is a possible abnormality seen on ECG in hypoparathyroidism?
Prolonged QT interval
What are the treatment options for hypoparathyroidism?
calcium supplementation, calcitriol/vitamin D supplementation, recombinant PTH therapy, phosphate binders
What is the Parathyroid Rule?
In primary parathyroid conditions, calcium and phosphate will be opposite (one raised, one low).
In secondary parathyroid conditions, calcium and phosphate will trend the same way (both increase or decrease) - except in CKD
What is pseudo-hypoparathyroidism?
A condition where tissues become resistant to the action of PTH, resulting in symptoms of low calcium/low PTH, despite high plasma levels of PTH
What are the relative calcium and phosphate levels in pseudo-hypoparathyroidism?
Decreased calcium, increased phosphate
