CSMLS - Canadian Society for Medical Laboratory Science Clinical Chemistry: Endocrinology Questions and Answers — Questions and Answers
Question 1: A patient presents with hypertension, headaches, and palpitations. The physician suspects a pheochromocytoma. Which of the following laboratory tests is considered the most sensitive for confirming this diagnosis?
- Urinary vanillylmandelic acid (VMA)
- Plasma free metanephrines (Correct answer)
- Plasma catecholamines
- Total urinary catecholamines
Correct answer: Plasma free metanephrines
Plasma free metanephrines are metabolites of catecholamines (epinephrine and norepinephrine) and are produced continuously within a pheochromocytoma. Their measurement has the highest sensitivity for detecting these tumors, often considered the test of first choice. While other tests like urinary VMA and catecholamines are used, they have lower sensitivity compared to plasma free metanephrines.
Question 2: A patient is diagnosed with Graves' disease. What is the characteristic pattern of thyroid function test results expected in this patient?
- High TSH, High Free T4
- Low TSH, High Free T4 (Correct answer)
- Low TSH, Low Free T4
- High TSH, Low Free T4
Correct answer: Low TSH, High Free T4
Graves' disease is an autoimmune disorder where thyroid-stimulating immunoglobulins (TSI) bind to and activate the TSH receptor, leading to excessive thyroid hormone production (hyperthyroidism). This results in high levels of Free T4 and T3. The high levels of thyroid hormones, in turn, suppress the pituitary's production of TSH through negative feedback, leading to a low or undetectable TSH level.
Question 3: A newborn screening test for congenital adrenal hyperplasia (CAH) is positive. Which analyte is typically measured and expected to be significantly elevated in the most common form of CAH, 21-hydroxylase deficiency?
- Cortisol
- Androstenedione
- 17-hydroxyprogesterone (Correct answer)
- Deoxycorticosterone
Correct answer: 17-hydroxyprogesterone
In 21-hydroxylase deficiency, the most common cause of CAH, the enzyme needed to convert 17-hydroxyprogesterone to 11-deoxycortisol (a precursor to cortisol) is deficient. This block in the steroid synthesis pathway leads to the accumulation of 17-hydroxyprogesterone, which is then shunted towards androgen production. Therefore, a markedly elevated level of 17-hydroxyprogesterone is the key diagnostic finding.
Question 4: A physician is evaluating a patient for Cushing's syndrome. Which of the following initial screening tests would be most appropriate to order?
- Random serum cortisol
- High-dose dexamethasone suppression test
- 24-hour urinary free cortisol (Correct answer)
- Serum ACTH
Correct answer: 24-hour urinary free cortisol
The recommended initial screening tests for Cushing's syndrome include the 24-hour urinary free cortisol, late-night salivary cortisol, and the low-dose dexamethasone suppression test. A 24-hour urinary free cortisol measurement reflects the cortisol production over a full day, overcoming the issue of diurnal variation that makes a random serum cortisol unreliable. High-dose suppression tests and ACTH levels are typically used later to determine the cause of Cushing's syndrome, not for initial screening.
Question 5: A patient is admitted with severe hyponatremia, low serum osmolality, and inappropriately concentrated urine (high urine osmolality). The patient is euvolemic. These findings are most consistent with which of the following conditions?
- Diabetes Mellitus
- Adrenal Insufficiency
- Diabetes Insipidus (DI)
- Syndrome of Inappropriate Antidiuretic Hormone (SIADH) (Correct answer)
Correct answer: Syndrome of Inappropriate Antidiuretic Hormone (SIADH)
SIADH is characterized by excessive release of ADH, which causes the kidneys to retain too much water. This leads to dilutional hyponatremia (low serum sodium), low serum osmolality, and the excretion of concentrated urine (high urine osmolality) because the body is not appropriately excreting free water. In contrast, Diabetes Insipidus involves a lack of ADH action, leading to hypernatremia and dilute urine.
Question 6: Which of the following is an important pre-analytical consideration when interpreting thyroid function tests during pregnancy?
- TSH levels are expected to be higher throughout pregnancy.
- Only total T4 should be used as free T4 is unreliable.
- Trimester-specific reference intervals should be used for TSH and free T4. (Correct answer)
- Thyroid peroxidase antibodies are always negative in a normal pregnancy.
Correct answer: Trimester-specific reference intervals should be used for TSH and free T4.
During pregnancy, physiological changes significantly alter thyroid hormone levels. For example, hCG can weakly stimulate the TSH receptor, leading to lower TSH levels, particularly in the first trimester. Additionally, estrogen increases thyroxine-binding globulin, affecting total hormone levels. Therefore, it is crucial to interpret TSH and free T4 results using trimester-specific reference intervals to accurately assess thyroid status.
CSMLS - Canadian Society for Medical Laboratory Science Clinical Chemistry: Endocrinology Questions and Answers
A patient presents with hypertension, headaches, and palpitations.
The physician suspects a pheochromocytoma.
Which of the following laboratory tests is considered the most sensitive for confirming this diagnosis?