Disorders of Ca and P metabolism Flashcards
1
Q
What is the Parathyroid anatomy?
A
- very small glands, sit in the neck - come in pairs
- behind the thyroid - can be ectopic
- ductless glands - chief cells put their output into the blood stream that perfuses the PT glands
- develop from pharyngeal poyches - superior from 4th arch and inferior from third
- thymus also develops from third, and when it descends, it drags the inferior pair with it
2
Q
What is the process of Vitamin D formation?
A
- UV hits the skin, converting 7-dehydrocholesterol into Vit D
- goes to the liver where it gets a hydroxyl group added on C25 and then to the kidney where another hydroxyl group is added to C1 (via 1alpha-hydroxylase)
- makes active VitD (1,25 dihydroxy vit d)
3
Q
What does Vitamin D do?
A
- acts in the gut to absorb more calcium
- acts on the bone to potentiate PTH, allowing more calcium resorption from the bone
- acts on bone - decreases calcium excretion by increasing calcium reabsorption in PCT
- also has negative feedback system where it inactivates 1alpha-hydroxylase and PTH.
4
Q
What does PTH do?
A
- acts mainly in the kidney - increases calcium reabsorption in DCT and decreases phosphate reabsorption
- On bone it causes calcium release
- minor effect on gut
5
Q
What happens in high calcium levels?
A
- inhibition of PTH release through Calcium sensor on the cell
- decrease in alpha-hydroxylation of VitD
- Stimulation of production of calcitonin - produced in C cells of the thyroid
- inhibits the action of PTH on the bone - only hormone that decreases calcium levels
6
Q
What are the causes of hypercalcaemia?
A
- causes divided into ones where PTH levels are high/low
- Most common is Hyperparathyroidism (PTH high)
- Very rarely a cancer can produce PTH too
- When PTH is low, it can be bony infiltration of a cancer
- High Vit D levels - exogenous, granulomatous disease or William’s syndrome
- Increased bone turnover - 9easier to break down bone than build it up again
7
Q
What are the causes of Hyperparathyroidism?
A
- Primary - tumour becomes autonomous - loses ability to switch off PTH production when Ca is high (PTH high, Ca high)
- Secondary - low calcium (due to Kidney failure, malabsorption, VitD deficiency), have to increase PTH levels (PTH high, Ca low)
- Tertiary - had secondary hyperparathyroidism then treat it (VitD replacement or renal transplant). PT gland used to giving lots of PTH for so long, it cannot regulate its output . (PTH high, Ca high)
8
Q
How do cancers cause Hypercalcaemia?
A
- PTH production (small cell lung cancer, or neuroendocrine tumour)
- PTH-rp production (lung, lumphoma, multiple myeloma)
- Osteoclast activating factor (lymphoma and multiple myeloma)
- Metastatic solid tumours (lung, breast, kidney, prostate)
9
Q
How can we miss multiple myeloma in radiotracer scans?
A
- Radiotracer put into bones, shows up where bony deposits are
= with myeloma, scan can be completely negative, but patient has lots of cancers
10
Q
What is a granulomatous disease?
A
- E.g. Sarcoidosis, TB, Berylliosis, Fungal infections
- have big pathogens in the body
- macrophages invade, coat it and encapsulate it in a granuloma
- TB is a chronic infection and you cannot clear it
- Berillyum is a large atom so cannot get rid of it, put a granuloma round it
- Macrophages also express 1a-hydroxylase and so can activate VitD
- cannot be regulated - only regulated by kidney
11
Q
What are some other causes of Hypercalcaemia?
A
- immobilisation (turn over becomes fast)
- recovery from renal transplant (calcium sensing receptor doesnt work - High Ca, High PTH)
- Familial hypocalcuric hypercalcaemia - Chr3q21 - autosomal dominant
- Milk-alkali syndrome (chalk deposits from taking antacids and drinking milk)
- Thiazide diuretics - work in DCT - block calcium excretion
12
Q
What are some symptoms of Hypercalcaemia?
A
- STONES, BONES and PSYCHIC MOANS
- band of calcium on cornea - band keratopathy
- Shortened QT interval
- Subarticular erosions
- neural depression
- slow GI motility
- thirst
13
Q
What investigations would you carry out to identify Hypercalcaemia?
A
- Mg, VitD, PTH, Urine, X-ray (lung cancer/sarcoidosis (net pattern of shadowing)), ACE levels (high in sarcoidosis)
- Myeloma - suppression of B cells (band in blood), produces bence jones proteins, bony lesions
14
Q
How do we decide if someone should have PT surgery?
A
- young patient - less than 50
- Ca lower than 3
- Dexa scan - if there is osteoperosis
15
Q
How do we localise the PT glands for surgery?
A
- SestaMIBI scan - tracer taken up lasts longer in PT than thyroid. Scan after 4/6 hours and whatever lights up is the PT glands
- Technesium thalium uptake scan - Thalium only taken up by thyroid, technesium taken up by both. Use both tracers, then you subtract one picture from the other.
- Ultrasound
