A 34-year-old woman comes to the clinic for an acute visit. Four weeks ago, she was diagno… — USMLE Step 3

A $34$ -year-old woman comes to the clinic for an acute visit. Four weeks ago, she was diagnosed with Graves disease and was started on methimazole and propranolol. She reports that her palpitations and hand tremors have significantly improved. However, yesterday she developed a mild sore throat and a subjective fever.

Her temperature is $37.2^\circ\text{C}$ ( $99.0^\circ\text{F}$ ), blood pressure is $118/76\text{ mmHg}$ , pulse is $82/\text{min}$ , and respirations are $14/\text{min}$ . Physical examination shows mild pharyngeal erythema without tonsillar exudates, palatal petechiae, or cervical lymphadenopathy. Her thyroid gland is diffusely enlarged, smooth, and non-tender.

Laboratory evaluation reveals the following:

Laboratory Parameter	Result	Reference Range
Leukocyte count	$3,400/\mu\text{L}$	$4,000 - 11,000/\mu\text{L}$
Absolute neutrophil count	$1,600/\mu\text{L}$	$1,500 - 8,000/\mu\text{L}$
Hemoglobin	$13.2\text{ g/dL}$	$12.0 - 15.5\text{ g/dL}$
Platelet count	$240,000/\mu\text{L}$	$150,000 - 400,000/\mu\text{L}$
Thyroid-stimulating hormone (TSH)	$<0.01\text{ }\mu\text{U/mL}$	$0.4 - 4.2\text{ }\mu\text{U/mL}$
Free thyroxine (T4)	$1.9\text{ ng/dL}$	$0.8 - 1.8\text{ ng/dL}$

Which of the following is the most appropriate next step in the management of this patient?

A
Discontinue methimazole and initiate propylthiouracil
B
Discontinue methimazole and admit the patient for intravenous cefepime
Continue methimazole and provide supportive careAnswer
D
Discontinue methimazole and refer for radioactive iodine ablation
E
Continue methimazole and order a bone marrow aspiration and biopsy

Answer

Continue methimazole and provide supportive care

The correct action is to continue methimazole and provide supportive care. Methimazole and propylthiouracil (thionamides) carry a rare but life-threatening risk of agranulocytosis (ANC

< 500/\mu\text{L}

), typically occurring within the first 90 days of therapy. Because the onset is idiosyncratic and sudden, routine CBC monitoring is not recommended. Instead, patients are instructed to seek immediate evaluation and obtain a CBC if they develop a fever or sore throat. In this patient, the total white blood cell count is mildly low (

3,400/\mu\text{L}

), but her absolute neutrophil count is normal (

1,600/\mu\text{L}

). This rules out agranulocytosis. Mild transient leukopenia with a preserved ANC is observed in up to 12% of patients with untreated or treated Graves disease. It is a benign condition (often due to the underlying autoimmune process itself) and is not a precursor to agranulocytosis. Therefore, her thionamide therapy should not be interrupted, and she should simply receive supportive care for her uncomplicated viral pharyngitis.

Step-by-Step Solution

Evaluate the patient's clinical presentation and the indication for laboratory testing.

The patient, who is on methimazole, presents with a sore throat and fever, appropriately prompting a complete blood count to rule out thionamide-induced agranulocytosis.

Agranulocytosis is a rare but life-threatening complication of thionamides that typically presents with sudden infectious symptoms within the first 90 days of therapy.

Interpret the laboratory findings with a specific focus on the leukocyte differential.

The total leukocyte count is mildly decreased (

3,400/\mu\text{L}

), but the absolute neutrophil count (ANC) is well within the normal range (

1,600/\mu\text{L}

Agranulocytosis is strictly defined as an ANC

< 500/\mu\text{L}

. The presence of a normal ANC definitively rules out this condition.

Determine the appropriate treatment intervention based on the established diagnosis.

The patient's mild leukopenia is benign, and the methimazole should be continued without interruption while providing symptomatic relief for the viral upper respiratory infection.

Mild, transient leukopenia (WBC

< 4,000/\mu\text{L}

with preserved ANC) is seen in up to 12% of patients with Graves disease due to autoimmune margination or a mild drug effect; it does not progress to agranulocytosis and does not require drug cessation.

Key Concept

Patients taking thionamides who develop infectious symptoms must be evaluated with a CBC; if the absolute neutrophil count (ANC) is normal (

\geq 1,500/\mu\text{L}

), the drug should be continued, as mild leukopenia is a common, benign feature of Graves disease.

Estimated Time:1m 30s

Question

Answer

Step-by-Step Solution

Key Concept

Common Mistakes

Topics

More questions from this topic