MLT GlobeChemistry: Endocrinology – page 5
123 Chemistry MCQs on Endocrinology with answers and explanations.
Why is 24-hour urine free cortisol more useful than a single plasma total cortisol when screening for Cushing syndrome?
Total cortisol varies with time of day and CBG, which estrogens raise. UFC reflects unbound cortisol produced over 24 hours.
A neonate with ambiguous genitalia has low cortisol and very high 17-hydroxyprogesterone. What is the most likely diagnosis?
Blocked cortisol synthesis raises ACTH, and precursors are shunted to androgens. Cortisol is low despite obvious adrenal overactivity, so 17-OHP is the key test.
Hypercortisolism has been confirmed. Which test best separates ACTH-dependent causes from a primary adrenal tumour?
Suppressed ACTH points to an adrenal source, and normal or high ACTH to a pituitary or ectopic source. The other tests confirm hypercortisolism but do not localise it.
How can primary adrenal insufficiency be told apart from secondary (pituitary) insufficiency?
Adrenal failure removes feedback, so ACTH rises and causes pigmentation. In pituitary failure ACTH is inappropriately low.
Which statement about plasma catecholamines is correct?
Catecholamines come from tyrosine in the adrenal medulla and sympathetic nerves, and most are excreted as metabolites (VMA, metanephrines). Samples need standardised rest.
Why is a 24-hour urine collection used for catecholamines instead of a single plasma sample?
Phaeochromocytomas often release hormones in bursts, which a timed collection captures. Stress, exercise and drugs still raise results, so specificity is limited.
Which method is used today to measure fractionated catecholamines and metanephrines?
HPLC-ECD and increasingly LC-MS/MS separate adrenaline, noradrenaline and dopamine with high specificity. The fluorometric and radioenzymatic methods are obsolete.
Growth hormone is secreted in pulses. Which analyte gives a stable, integrated measure of GH secretion?
IGF-1 is made in the liver in response to GH, has a long half-life on IGFBP-3, and varies little over the day. Levels are interpreted against age-specific ranges.
Which statement about ACTH is FALSE?
Aldosterone is controlled mainly by angiotensin II and potassium, so it stays normal in ACTH deficiency. ACTH drives cortisol and adrenal androgens and is trophic to the cortex.
Which statement about T3 and T4 is correct?
The thyroid secretes mostly T4. About 80% of T3 is made in tissues by deiodinases. T3 is several times more potent, and T4 far exceeds T3 in plasma.
A pregnant woman has raised total T4 but normal free T4 and TSH. What is the explanation?
Estrogen raises TBG, so more T4 is bound. Only free hormone is active and it stays normal, so the patient is euthyroid.
Which handling is recommended for a plasma ACTH specimen?
Plasma proteases rapidly degrade ACTH and it sticks to glass. Pre-chilled EDTA tubes (plastic), cold centrifugation and freezing prevent falsely low results.
A thyroid panel shows TSH below the detection limit with raised free T4 and total T3. Which interpretation fits best?
High thyroid hormones with suppressed TSH indicate autonomous thyroid overactivity. A pituitary TSH-oma would give normal or raised TSH; binding-protein excess leaves free T4 normal.
Failure of serum cortisol to suppress after a 1 mg overnight dexamethasone test suggests:
Dexamethasone normally suppresses ACTH and so cortisol. Persistent cortisol (above about 1.8 µg/dL) suggests autonomous cortisol production. Addison disease is investigated with an ACTH stimulation test.
A patient has low TSH and low free T4 with symptoms of hypothyroidism. The most likely cause is:
In central hypothyroidism the pituitary fails to produce TSH, so both TSH and free T4 are low or inappropriately normal TSH is seen. Hashimoto thyroiditis gives high TSH.
A patient with palpitations has suppressed TSH, normal free T4 and raised free T3. This is called:
In T3 toxicosis only T3 is raised, often early in Graves disease or with a toxic nodule. Free T3 should be measured when TSH is suppressed and free T4 is normal.
A critically ill ICU patient has low free T3, normal-to-low free T4 and normal TSH. The most likely interpretation is:
Severe illness reduces peripheral T4-to-T3 conversion and increases reverse T3. Thyroid tests are best not interpreted in acute illness unless dysfunction is strongly suspected.
After total thyroidectomy for differentiated thyroid cancer, thyroglobulin is used as a tumor marker. Which result must be checked with it?
Thyroglobulin antibodies can cause falsely low thyroglobulin results in immunometric assays, so they must be measured together. Calcitonin is the marker for medullary thyroid carcinoma.
In a short Synacthen (cosyntropin) stimulation test, a normal response is:
Synthetic ACTH should stimulate a normal adrenal to raise cortisol above a defined threshold (assay-dependent, often around 400–500 nmol/L). A poor response suggests adrenal insufficiency.
Late-night salivary cortisol is used as a screening test for Cushing syndrome because:
Normal cortisol is lowest around midnight; loss of this circadian nadir is sensitive for Cushing syndrome. Salivary cortisol reflects free, not bound, cortisol.