MLT GlobeChemistry: Endocrinology – page 6
123 Chemistry MCQs on Endocrinology with answers and explanations.
A woman with galactorrhoea has a very large pituitary mass but prolactin is only mildly raised. Which step should the lab take?
Very high prolactin can saturate both antibodies in a two-site immunoassay, giving a falsely low result (high-dose hook effect). Dilution reveals the true high concentration.
An asymptomatic woman has repeatedly raised prolactin with regular menses. Which test should be done next?
Macroprolactin (prolactin–IgG complex) is biologically inactive but detected by many immunoassays. PEG precipitation identifies it and avoids unnecessary imaging.
A patient taking high-dose biotin supplements has a sandwich (two-site) immunoassay using streptavidin–biotin capture. The result is expected to be:
Excess biotin blocks streptavidin binding, so less labelled complex is captured, giving falsely low sandwich assay results (e.g. TSH). In competitive assays (e.g. free T4) results are falsely high.
Heterophile antibodies in a patient sample most commonly cause which error in two-site immunometric assays?
Heterophile and human anti-animal antibodies can link capture and detection antibodies without analyte, giving false positives. Blocking reagents, dilution non-linearity or another method help detect them.
Total testosterone may be misleading in an obese man with low sex hormone-binding globulin (SHBG). The better assessment is:
Low SHBG lowers total testosterone without affecting free hormone. Free testosterone, calculated from total testosterone, SHBG and albumin, gives a better picture.
A 2-year-old has an abdominal mass. Which urine tests are most useful to support neuroblastoma?
Most neuroblastomas secrete catecholamines, so urine HVA and VMA (expressed per creatinine) are raised. 5-HIAA is used for carcinoid (serotonin-secreting) tumors.
A patient is collecting a 24-hour urine for 5-HIAA. Which instruction is most important?
These foods are rich in serotonin and can falsely raise urine 5-HIAA. The test screens for serotonin-producing neuroendocrine (carcinoid) tumors.
A patient with small cell lung cancer has sodium 122 mmol/L, serum osmolality 255 mOsm/kg, urine osmolality 480 mOsm/kg and urine sodium 50 mmol/L. He is euvolemic. The most likely diagnosis is:
Low serum osmolality with inappropriately concentrated urine and high urine sodium in a euvolemic patient fits SIADH. Primary polydipsia gives dilute urine (< 100 mOsm/kg).
Which single test is best for screening a patient suspected of acromegaly?
IGF-1 reflects integrated GH secretion and is stable through the day. Random GH is pulsatile and unreliable; failure of GH suppression during an OGTT is used to confirm.
A tall young man with small testes and infertility has high LH and FSH and low testosterone. Which karyotype is most likely?
Klinefelter syndrome (47,XXY) causes primary testicular failure: low testosterone with high gonadotropins. Kallmann syndrome has low LH and FSH.
In many newborn screening programmes, which analyte in a dried heel-prick blood spot is used to detect congenital hypothyroidism?
Most programmes measure TSH (sometimes with T4) on dried blood spots; a high TSH suggests primary congenital hypothyroidism. Samples taken too early may show a physiological TSH surge.
A tall 12-year-old boy is growing very fast and has a pituitary tumor secreting excess growth hormone. This condition is called:
Excess GH before the growth plates close causes gigantism. After closure, excess GH causes acromegaly. Cretinism is congenital hypothyroidism.
A patient takes levothyroxine for primary hypothyroidism. Which test is used to check that the dose is correct?
In primary hypothyroidism the pituitary is normal, so TSH is the most sensitive guide to thyroxine replacement; the dose is adjusted to keep TSH within the reference range.
A patient with Addison disease (primary adrenal insufficiency) is most likely to show which electrolyte pattern?
Lack of aldosterone reduces sodium reabsorption and potassium excretion, giving hyponatremia and hyperkalemia. High sodium with low potassium is typical of aldosterone excess.
In Cushing disease (ACTH-secreting pituitary adenoma), what is the usual response to a high-dose dexamethasone suppression test?
Pituitary adenomas keep partial glucocorticoid feedback, so high-dose dexamethasone suppresses them. Ectopic ACTH tumours and adrenal tumours do not suppress.
Which statement about growth hormone deficiency testing is correct?
GH secretion is pulsatile, so random levels mislead. Deficiency is confirmed by stimulation tests (insulin tolerance, arginine, glucagon). GH is usually lost early in pituitary failure.
How do current 'intact PTH' immunoassays reduce interference from inactive C-terminal fragments?
Two-site sandwich assays detect only molecules carrying both epitopes, which is mainly PTH(1–84). C-terminal fragments build up in renal failure and would falsely raise single-site results.
Which procedure is the most specific for confirming a pituitary source of ACTH?
A central-to-peripheral ACTH gradient on IPSS, especially after CRH, confirms Cushing disease. Dexamethasone tests overlap between pituitary and ectopic sources.
Which statement correctly describes reverse T3 (rT3)?
Inner-ring deiodination of T4 in tissues yields inactive rT3. In non-thyroidal (euthyroid sick) illness rT3 rises because its clearance falls, not decreases.
A patient on biotin shows suppressed TSH with high free T4 on streptavidin–biotin assays but no symptoms. The correct approach is:
Biotin interference can mimic hyperthyroidism (falsely low TSH, falsely high free T4). Stopping biotin for a suitable period (often at least 2–3 days for high doses) or using another platform resolves it.