24 – Parathyroid Gland Flashcards
(27 cards)
1
Q
What is total serum calcium composed of?
A
- Free (‘ionized’) calcium: 50%
- Protein bound calcium: 40%
- Complexed calcium: 10%
2
Q
What are the main organs involved in calcium homeostasis?
A
- Intestines: absorbed in the ileum (mediated by Vit D/calcitriol: except in horses)
- Bone: stored and released (Ca release by action of PTH; inhibited by calcitonin)
- Kidneys: excreted (PTH and Vit promote Ca reabsorption)
3
Q
What mediates phosphate homeostasis?
A
- Released from bone (stimulated by PTH)
- Absorbed in small intestine (enhanced by Vit D)
- *excreted in urine (increased by PTH); remainder in feces
4
Q
Adult ruminants and phosphate homeostasis?
A
- P mostly excreted in saliva (NOT urine); remainder in feces
- Lactation and fetal development may be significant source of phosphate loss
5
Q
Phosphate and transcellular shifting
A
- P moves INTO cells with insulin and alkalemia
- P moves OUT of cells with acidemia (secretory metabolic acidosis
6
Q
What happens with parathyroid hormone abnormalities?
A
- Ca and P tend to move in opposite directions
- Ex. primary hypoparathyroidism
o Decreased Ca
o Increased P
7
Q
What happens with Vit D abnormalities?
A
- Ca and P tend to move in same direction
- Excess Vit D
o Increased Ca and P - Vit D deficiency
o Decreased Ca and P
8
Q
When might you see hypocalcaemia?
A
- HYPOALBUMINEMIA
o Decreased bound Ca? - Primary hypoparathyroidism
- Nutritional secondary hyperparathyroidism
- Hypovitaminosis D
- Chronic renal failure (cats, most dogs, cattle)
- Malabsorption with PLE
- Anorexia in cattle
- Ethylene glycol toxicity
- Pregnancy, parturient or lactational hypocalcaemia/eclampsia
- Acute pancreatitis
9
Q
When might you see hypercalcemia? (HOGSINYARD)
A
- Hyperproteinemia
- Hyperparathyroidism
- Osteolysis
- Granulomatous disease
- Spurious (lab or sampling error)
- *IDIOPATHIC (cats) or iatrogenic
- NEOPLASIA: apocrine gland ACA, LSA
- Youth (puppy <6mo old)
- Addison’s (hypoadrenocorticism)
- Renal disease (horse, young dogs)
- Excess (hypervitaminosis D)
10
Q
What are the clinical signs with hypercalcemia? And values?
A
- PU/PD, lethargy, inappetence, weakness
- See with tCa >3.5mmol/L or iCa >1.6mmol/L
- Life threatening with tCa >4.5mmol/L or iCa >2.2mmol/L
- ***Metastatic calcification when product of Ca x P exceeds 6
11
Q
What are the clinical signs with hypocalcaemia? And values?
A
- Muscle tremors and fasciculations, facial rubbing, muscle cramping, stiff gait, seizures
- See with tCa < 1.6mmol/L or iCa <0.8mmol/L
- Cattle: recumbency with tCa <1.5mmol/L and life-threatening with tCa < 0.9 mmol/L
12
Q
Humoral hypercalcemia of malignancy
A
- Accounts for hypercalcemia in >50% of canine cases
- Paraneoplastic syndrome usually due to secretion of PTH related protein (PTHrp)
o Stimulates the same receptors as PTH
o Increased iCa, low or low-normal P, decreased PTH, increased PTHrp - Occasionally due to tumor secretion of Vit D or osteoclastogenic cytokines
13
Q
What are the most common tumours? (humoral hypercalcemia of malignancy)
A
- Lymphoid neoplasms of T-cell origin
- Apocrine gland adenocarcinoma of the anal sac
14
Q
Idiopathic hypercalcemia in cats
A
- Young to middle aged, DLH over-represented
- (free) hypercalcemia w/o known underlying cause
- Initially asymptomatic (months to years) but may progress
o Weight loss, anorexia, vomiting, constipation, PU/PD
o Calciuresis may lead to calcium oxalate urolithiasis - *diagnosis of exclusion
- Treatment: diet, bisphosphonates (if symptomatic)
15
Q
Idiopathic hypocalcaemia in cats: diagnosis of exclusion
A
- Hypercalcemia is mild to moderate (iCa 1.4-1.9 mmol/L, total Ca <3.75mmol/L)
- Serum P normal, PTH low or low-normal, PTHrp negative, 25-VitD is normal
16
Q
Ca and CKD in cats/dogs
A
- Most have normal serum Ca
- Hypocalcaemia may occasionally be seen
- Hypercalcemia is uncommon and tends to be mild
17
Q
What is hypocalcemia in some small animals with CKD driven by?
A
- Progressive hyperphosphatemia
- Leads to defective VitD metabolism by inhibiting renal 1alpha-hydroxylase
o Decreased calcitriol
o Decreased renal/intestinal Ca absorption - Activates FGF-23 (due to high P?)
o Suppression of both calcitrol/PTH (initially) - High P complexes with Ca forming crystals that deposit in tissues
18
Q
What might hypocalcemia in some small animals with CKD lead to?
A
- Renal secondary hyperparathyroidism
- Low Ca, high P cause progressive increase in PTH synthesis
- High PTH increases bone resorption and may lead to fibrous osteodystrophy (CKD-mineralized bond disease (MBD))
- P also released from bone and hyperphosphatemia worsens
- Progressive soft tissue mineralization
19
Q
Hypercalcemia is uncommon in some small animals with CKD?
A
- Often reflects increase in ‘complexed’ fraction (these increase in renal failure)
o Ca binds to citrates/phosphates - True hypercalcemia (increased free Ca) may reflect reduced renal clearance
20
Q
Concurrent hypercalcemia and CKD: which came first?
A
- CKD may result in hypercalcemia, but hypercalcemia may lead to renal failure via mineralization of renal tissue
- Cats/dogs: hyperCa more likely to be the cause of renal failure rather than the result of renal failure
o SO investigate other causes of hypercalcemia first - Horses: more likely to have hyperCa b/c of CKD
21
Q
Horses lack renal 1alpha-hydroxylase (usually: converts Vit D to active form=calcitriol)
A
- Likely due to high dietary content of Ca
- Do NOT rely on Vit D for intestinal Ca absorption
o Can develop hypocalcaemia with intestinal disease
22
Q
What is the major mechanism of elimination of dietary calcium in horses?
A
- Renal excretion
- Chronic renal disease often leads to hypercalcemia
- Often accompanied by hypophosphatemia
23
Q
What is the approach to abnormal serum total calcium?
A
- Rule out hyper/hypoproteinemia
- Rule out age and obvious explanations
- Recheck Ca to determine if persistent finding
- Measurement of free calcium (blood gas)
- Measurement of PTH (panel with iCa, PTH +/- PTHrp)
24
Q
How can you rule out hyper/hypoproteinemia?
A
- Decreased/increased protein binding typically only causes MILD changes in total Ca
25
If a patient has hypercalcemia what should you do?
- Assess patient for neoplastic disease
- Ex. check anal glands, lymph nodes, imaging studies
26
Nutritional secondary hyperparathyroidism
- Uncommon, but can occur in all species
- Imbalance of Ca and P in diet
o P:Ca > 3:1 or diet high in oxalates
o Ex. grain diets in horses OR high organ meat diet in carnivores
- Low free iCa stimulates PTH
o Causes release of Ca from bone stores (osteolysis/osteopenia)
o Lameness, bone pain, “big head” (fibrous osteodystrophy), pathological fractures
27
What are the common lab abnormalities with nutritional secondary hyperparathyroidism?
- Ca/P may be normal (or see mild decrease Ca and increase P)
- +/- increased ALP (bone isoform)
- Elevated PTH
