During an outpatient evaluation, a 28-year-old female patient reports experiencing profoun… — USMLE Step 2 CK

During an outpatient evaluation, a 28-year-old female patient reports experiencing profound anxiety, palpitations, and a $5$ -kg ( $11$ -lb) weight loss over the past two months. She states she recently began taking various unprescribed "fat-burner" dietary supplements purchased online to prepare for a fitness competition. She denies any neck pain, recent viral illnesses, or changes in her vision. Her temperature is $37.0^{\circ}$ C ( $98.6^{\circ}$ F), blood pressure is $135/75$ mm Hg, and pulse is $114$ /min. Physical examination reveals a fine resting tremor of the outstretched hands, warm and moist skin, and brisk deep tendon reflexes. Ophthalmic examination shows no proptosis, lid lag, or periorbital edema. The thyroid gland is not palpable, and there is no cervical lymphadenopathy. Laboratory studies reveal:

Thyroid-stimulating hormone (TSH): $< 0.02$ $\mu$ U/mL
Free thyroxine (T4): $3.2$ ng/dL

Which of the following additional findings is most likely present in this patient?

Decreased serum thyroglobulin concentrationAnswer
B
Elevated serum thyroglobulin concentration
C
Diffusely increased radioactive iodine uptake
D
Increased color Doppler flow on thyroid ultrasound
E
Elevated thyroid-stimulating immunoglobulin levels

Answer

Decreased serum thyroglobulin concentration

This patient's presentation of thyrotoxicosis symptoms, suppressed TSH, elevated free T4, use of unprescribed weight-loss supplements, and a non-palpable thyroid gland is highly suggestive of factitious thyrotoxicosis (exogenous thyroid hormone ingestion). Many unregulated 'fat burners' contain animal thyroid extract or synthetic levothyroxine. In this condition, the exogenous hormone suppresses pituitary TSH secretion. The lack of TSH stimulation causes the endogenous thyroid gland to become inactive. Thyroglobulin is a protein precursor produced exclusively by the thyroid gland during the normal synthesis of endogenous thyroid hormones. Because the gland is inactive in exogenous thyrotoxicosis, serum thyroglobulin concentrations will be remarkably decreased. This key laboratory finding distinguishes exogenous thyrotoxicosis from destructive thyroiditis (e.g., subacute or silent thyroiditis), which also presents with low radioactive iodine uptake but features an elevated thyroglobulin level due to the release of preformed hormone and cellular debris from inflamed follicles.

Step-by-Step Solution

Analyze the clinical presentation and initial laboratory results.

The patient has clinical thyrotoxicosis with a suppressed TSH and elevated free T4, confirming primary hyperthyroidism or exogenous thyroid hormone excess.

Establishing the biochemical presence of thyrotoxicosis is the first step in the diagnostic algorithm.

Evaluate the physical examination findings and history for etiology.

The patient lacks a palpable goiter, has no eye findings (proptosis), and is taking unregulated "fat-burner" supplements, pointing strongly toward exogenous thyroid hormone ingestion (factitious thyrotoxicosis).

Differentiating the cause of thyrotoxicosis dictates the expected ancillary test results.

Determine the expected thyroglobulin level based on the etiology.

Because the excess thyroid hormone is coming from an exogenous source, the endogenous thyroid gland is suppressed by the low TSH. Therefore, thyroglobulin (a byproduct of endogenous hormone synthesis) will be decreased.

Thyroglobulin levels differentiate exogenous thyrotoxicosis (low) from endogenous hyperthyroidism or destructive thyroiditis (both high).

Key Concept

Differentiation of thyrotoxicosis etiologies using serum thyroglobulin

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