Here are a few quick notes on lab studies of endocrine diseases. I focused on the most important, sensitive, and specific items you should look for when suspecting an endocrine disease. In the USMLE, usually the question will offer a few lab results. Sometimes one result can be specific to a disease. For instance, if you find a mention of ‘thyroid stimulating immunoglobulins; TSI’ in the question, then without a doubt the diagnosis is Graves disease. So these notes are more of a high-yield guide, rather than a complete text in its own. Extracted mostly from First Aid for Step 2, Kaplan, and other sources.

Reference:
» means ‘leads to’.
+ (++) means ‘increase of’.
- (- -) means ‘decrease of’.
ttt means ‘treatment’.

¶ ADRENAL

CUSHING SYNDROME

• Hyperglycemia, glycosuria, hypokalemia (dt Na/H2O retension)
• ACTH is high only in ACTH-secr. adenomas (pituitary or ectopic). Normal/low in iatrogenic or adrenal causes (+cortisol » -ACTH)
• DHEA (secr. from adr. cortex) is relatively low in iatrogenic causes. High in all other causes.
• Screen; Low-dose dexamethasone suppression test (to – ACTH » – cortisol). +ve if cortisol is persistently elevated
• Confirm; 24-hr free urine cortisol (high)
• Diff.; High-dose dexamethasone suppression test (to – ACTH » – cortisol). Cortisol is suppressed in pituitary adenomas, but not in ectopic ACTH tumors (feedback invalid) or adrenal cortisol-secr. tumor

ADRENAL INSUFFICIENCY

• Hyponatremia, » hyperkalemia (+acidosis), eosinophilia
• ACTH; high in 1ry dse (Addison), low in 2ry dse (pituitary dysfn)
• ACTH inj. » cortisol remains low in 1ry dse (Addison), high in 2ry dse (pituitary dysfn)

HYPERALDOSTERONISM

• Polyuria! (2ry to hypokalemic nephropathy)
• Elevated 24-hr urine aldostreone
• Hypernatremia, » hypokalemia (+alkalosis), low renin (++ aldosterone » – - renin)

CONGENITAL ADRENAL HYPERPLASIA

• Low cortisol (mild, dt hyperplasia)
• High ACTH
• High cortisol precursors
• High androgens (» sexual ch. abnormalities)

PHEOCHROMOCYTOMA

• 24-hr urine catecholamine metabolites; VMA, HVA, etc (high)

¶ GLUCOSE-RELATED

DIABETES MELLITUS

• At least ONE of the following plasma glucose conc. tests is required for the diagnosis:
Random: > 200 mg/dl (+ symptoms)
Fasting: > 126 mg/dl (twice)
Postprandial: > 200 mg/dl (twice)
• Glucosuria, ketonuria supports diognosis
• HbA1c monitors efficacy of & compliance with ttt (goal is <7)

HYPERPARATHYROIDISM

• Very high PTH, » hypercalcemia, » hypercalciuria, » hypophosphatemia

ACROMEGALY

• High GH (unreliable)
• Screen; Insulin-like growth factor I (IGF-I) » high
• Confirm; Oral glucose tolerance test » GH remains high

¶ THYROID

HYPOTHYROIDISM

• High TSH (most sensitive in 1ry dse)
• Low total T4, free T4, free T3
• High ESR in thyroiditis
• Anti-TG ABs in Hashimoto

HYPERTHYROIDISM

• Low TSH (most sensitive in 1ry dse)
• High total T4, free T4, free T3
• TSI present in Graves