endocrinology (parathyroid glands) Flashcards
anatomy and physiology
describe the anatomy of the parathyroid glands
there are two pairs of parathyroid glands in cats:
- internal and external parathyroids located adjacent to the thyroid tissue
presence of ectopic parathryoid tissue is common
which cells within the parathyroid gland are responsible for PTH seceretion
the chief cells are the major cell type in the parathyroid and are responsible for secretion of parathyroid hormone (PTH)
how is blood calcium concentration regulated
regulation of blood calcium concentrations is under the influence of PTH, but also calcitonin and calcitriol
how PTH acts to increase serum calcium levels
PTH acts to increase serum calcium levels by:
- stimulation of calcium and phosphate resorption from bone
- increase renal calcium resorption and increase phosphate excretion from tubular fluid
- stimulate renal tubular 1-alpha-hydroxylase activity and thus increase production of calcitriol from the kidneys (indirectly increasing GI absorption of calcium and phosphate)
what is the main inhibitor for 1-alpha-hydroxylase activity
hyperphosphatemia
what affects PTH secretion from the chief cells
PTH secretion from the chief cells is affected by:
- low ionised calcium is the major PTH secretagogue though in some disease such as renal secondary hyperparathyroidism, the sensitivity of chief cells to calcium is altered thus there is an increased PTH secretion in response to normal calcium concentrations
- low calcitriol concentrations - increased phosphate concentrations also directly stimulate PTH secretion but this is the weakest mechanism affecting PTH secretion
how is calcium found in blood and what is the implication for calcium dosage
in blood, approximately 50% of the total calcium is present in the free ionised form, with about 40% protein bound (primarily albumin) and around 10% complexed
measurement of ionised calcium is always preferred as this accurately reflects calcium status (metabolically active form)
what is PTHrp
PTHrp is a PTH-related peptide that is frequently responsible for hypercalcemia of malignancy
what are the most commonly encountered disorders of the parathyroid gland
the most commonly encountered disorders of the parathyroid gland occur secondary to the presence of other diseases (e.g., nutritional secondary hyperparathyroidism, renal secondary hyperparathyroidism, iatrogenic following thyroid surgery, ….)
what are the major differential diagnoses of feline hypercalcemia
neoplasia
- lymphoma
- myeloproliferative disorders
- myeloma
- squamous cell carcinoma
hyperparathyroidism
hypoadrenocorticism
granulomatous diseases
hypervitaminosis D
CRF
spurious (e.g., hemolysis, lipemia)
idiopathic
what are the most common causes of feline hypercalcemia
the most common causes of feline hypercalcemia are said to be CKD, neoplasia and idiopathic hypercalcemia
what is important to consider when interpreting total calcium concentrations
it is essential to interpret total calcium concentrations in the light of albumin concentrations and confirmed by measurement of ionised calcium
what is hypercalcemia a risk factor for
hypercalcemia is a risk factor for calcium oxalate uroliths and a proportion of cats with hypercalcemia (15% in one study) will have signs of lower urinary tract disease due to CaOx uroliths as their presenting sings
