Endocrine, Physiology, USMLE Step 1 - Full Vignette with Extended Explanations

Описание: A 37-year-old pregnant woman presents at 10 weeks with severe nausea, weight loss, tremor, heat intolerance, and palpitations. Her lab findings include suppressed TSH, near-high free T4, and mildly elevated liver enzymes. Physical exam reveals brisk reflexes and a small, smooth thyroid gland without eye findings. How would you approach distinguishing the cause of her thyrotoxicosis in pregnancy? Which clinical and laboratory features are most helpful in guiding the next diagnostic step?

VIDEO INFO
Category: Endocrine, Physiology, USMLE Step 1
Difficulty: Hard - Advanced level - Challenges experienced practitioners
Question Type: Differential Testing
Case Type: Pregnant Patient

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QUESTION
A 37-year-old G2P1 woman at 10 weeks gestation presents with 10 days of severe nausea and vomiting with 3 kg weight loss, tremulousness, heat intolerance, and palpitations. She reports intermittent paresthesias in the first three digits, diagnosed 2 years ago as carpal tunnel syndrome. She has no known allergies....

OPTIONS
A. Measure maternal TSH receptor antibodies (TRAb, including thyroid-stimulating immunoglobulins) now, interpreting TSH and free T4 with trimester-specific reference ranges while providing supportive care for hyperemesis.
B. Obtain color Doppler thyroid ultrasonography to quantify diffuse hypervascularity and diagnose Graves disease in pregnancy without serologic testing.
C. Order an iodine-123 thyroid uptake and scan with abdominal shielding because modern doses are low and safe enough to differentiate Graves disease from hCG-mediated thyrotoxicosis in early pregnancy.
D. Trend serum hCG levels and repeat thyroid function tests in 4 weeks; if hCG declines but free T4 remains high, infer Graves disease and begin methimazole in the second trimester.

CORRECT ANSWER
A. Measure maternal TSH receptor antibodies (TRAb, including thyroid-stimulating immunoglobulins) now, interpreting TSH and free T4 with trimester-specific reference ranges while providing supportive care for hyperemesis.

EXPLANATION
In early pregnancy, very high human chorionic gonadotropin can stimulate the TSH receptor, lowering TSH and sometimes slightly increasing free T4 without autoimmunity-gestational transient thyrotoxicosis. Distinguishing this from Graves disease determines whether antithyroid drugs are indicated and whether fetal surveillance for maternal TSH-receptor antibodies is needed. The single best, pregnancy-safe discriminator is maternal TSH-receptor antibodies (TRAb, including thyroid-stimulating immunoglobulins). Per the 2017 American Thyroid Association pregnancy guideline, TRAb testing is appropriate when the diagnosis is uncertain, and trimester-specific reference ranges should be used to interpret TSH and free T4. Supportive management of hyperemesis (hydration, antiemetics) can be continued while awaiting results.

Color Doppler hypervascularity is nonspecific and cannot confidently separate hCG-mediated thyrotoxicosis from Graves. Radioiodine uptake and scanning is contraindicated in pregnancy and should not be used. Delaying discrimination by merely trending hCG and repeating thyroid tests in several weeks can postpone needed therapy in Graves and does not provide the important fetal risk information that a positive TRAb would confer. Therefore, measuring TRAb now-while interpreting thyroid function with trimester-specific cutoffs-is the most efficient and safe next test.


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