A 52-year-old woman is evaluated in the clinic due to new-onset palpitations, resting trem… — USMLE Step 2 CK

A $52$ -year-old woman is evaluated in the clinic due to new-onset palpitations, resting tremor, and heat intolerance over the past four weeks. She also reports a $3$ -kg ( $6.6$ -lb) unintentional weight loss despite maintaining a normal appetite. Her past medical history is unremarkable, and she takes no medications. Vital signs show a temperature of $37.1^\circ$ C ( $98.8^\circ$ F), blood pressure of $135/75$ mmHg, and heart rate of $112$ /min. Physical examination reveals warm, moist skin, a fine resting tremor in the outstretched hands, and a diffusely enlarged thyroid gland without discrete nodules. Cardiovascular examination confirms a regular tachycardia. No exophthalmos or pretibial myxedema is noted.

Initial laboratory studies show:

Laboratory test	Result	Reference range
Thyroid-stimulating hormone (TSH)	$0.03$ $\mu$ U/mL	$0.4-4.2$ $\mu$ U/mL
Free thyroxine (T4)	$1.4$ ng/dL	$0.8-2.7$ ng/dL

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

Measurement of serum triiodothyronine (T3) levelAnswer
B
Radioactive iodine uptake and scan
C
Measurement of thyroid-stimulating immunoglobulins
D
Initiation of methimazole therapy
E
Reassurance and repeat thyroid function tests in 6 weeks

Answer

Measurement of serum triiodothyronine (T3) level

The correct next step is the measurement of serum triiodothyronine (T3). This patient presents with overt clinical symptoms of hyperthyroidism (tachycardia, tremor, weight loss) and a suppressed TSH, but her Free T4 is surprisingly normal. This specific laboratory pattern can indicate either subclinical hyperthyroidism or T3 toxicosis. T3 toxicosis occurs when the thyroid gland autonomously produces excessive amounts of T3 rather than T4, which often happens in early Graves' disease or toxic nodular goiter. Because she is highly symptomatic, T3 toxicosis is strongly suspected and must be biochemically confirmed by measuring the serum T3 level before establishing the etiology with imaging or initiating antithyroid medications.

Step-by-Step Solution

Evaluate the clinical presentation and physical exam.

The patient exhibits classic symptoms of hyperthyroidism, including palpitations, tremor, weight loss, and tachycardia.

Identifying the clinical state dictates the urgency and direction of the laboratory workup.

Interpret the initial thyroid function tests.

The TSH is severely suppressed (

0.03

\mu

U/mL), but the Free T4 is within the normal reference range.

A suppressed TSH usually correlates with an elevated T4 in overt primary hyperthyroidism. A normal Free T4 requires further investigation.

Determine the next necessary diagnostic test.

Measure serum T3 to evaluate for T3 toxicosis.

In early hyperthyroidism (especially early Graves' disease or autonomous nodules), the thyroid may preferentially secrete T3 over T4. Normal T4 does not rule out thyrotoxicosis.

Key Concept

In a symptomatic patient with a suppressed TSH and a normal free T4, serum T3 must be measured to differentiate T3 toxicosis from subclinical hyperthyroidism before proceeding with imaging or treatment.

Estimated Time:1m 15s

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