A 34-year-old woman comes to the clinic for evaluation of a 4-week history of palpitations… — USMLE Step 2 CK

A $34$ -year-old woman comes to the clinic for evaluation of a $4$ -week history of palpitations, anxiety, sweating, and a $3$ -kg ( $6.6$ -lb) weight loss. She works as a medical assistant and reports high stress levels. She has no neck pain, dysphagia, or recent viral illnesses. Her temperature is $37.1^\circ\text{C}$ ( $98.8^\circ\text{F}$ ), blood pressure is $135/85\text{ mm Hg}$ , and pulse is $112\text{/min}$ . Physical examination reveals warm, moist skin and a fine resting tremor. The eyes appear normal without proptosis. The thyroid gland is normal in size, non-palpable, and non-tender.

Laboratory studies show:
Thyroid-stimulating hormone (TSH): $<0.01\text{ }\mu\text{U/mL}$
Free thyroxine (T4): $2.9\text{ ng/dL}$ (Normal: $0.9-1.7\text{ ng/dL}$ )

A radioactive iodine uptake (RAIU) scan reveals $1\%$ uptake at $24$ hours (Normal: $15\%-30\%$ ).

Which of the following is the most appropriate next step in evaluating this patient's condition?

Measurement of serum thyroglobulinCevap
B
Measurement of thyroid-stimulating immunoglobulins
C
Initiation of methimazole therapy
D
Ultrasound of the thyroid gland
E
Measurement of 24-hour urinary iodine excretion

Cevap

Measurement of serum thyroglobulin

The correct answer is measurement of serum thyroglobulin. This patient has thyrotoxicosis characterized by a suppressed TSH, elevated free T4, and a low radioactive iodine uptake (RAIU). A low RAIU indicates decreased de novo synthesis of thyroid hormone. The differential diagnosis includes destructive thyroiditis (e.g., painless/silent thyroiditis, subacute thyroiditis), iodine-induced thyrotoxicosis, or exogenous thyroid hormone intake. Serum thyroglobulin is a precursor protein synthesized by normal thyroid follicular cells and co-secreted with endogenous thyroid hormone. In destructive thyroiditis, preformed thyroglobulin is released from damaged follicles, resulting in elevated serum levels. Conversely, exogenous thyroid hormone administration suppresses TSH, which in turn suppresses endogenous thyroglobulin production, leading to low serum thyroglobulin levels. Because this patient works in the medical field (increasing access to medications) and lacks symptoms of viral subacute thyroiditis (like neck pain), distinguishing between painless thyroiditis and factitious thyrotoxicosis is critical. Measuring thyroglobulin is the definitive next step to make this distinction.

Adım Adım Çözüm

Identify the primary clinical syndrome.

The patient has clinical signs of thyrotoxicosis confirmed by a suppressed TSH and an elevated free T4.

Establishing the biochemical presence of primary thyrotoxicosis is the required first step before determining the underlying etiology.

Interpret the radioactive iodine uptake (RAIU) scan.

The RAIU is severely suppressed at 1%, which is well below the normal range.

A low RAIU indicates that the thyroid gland is not actively synthesizing new hormone, ruling out causes like Graves disease or toxic nodular goiter.

Formulate a differential diagnosis for low-uptake thyrotoxicosis.

The differential is narrowed to destructive thyroiditis (e.g., painless or subacute thyroiditis), iodine-induced thyrotoxicosis, or exogenous thyroid hormone intake (factitious thyrotoxicosis).

These conditions cause thyrotoxicosis through the release of preformed hormone or external ingestion, thereby suppressing TSH and halting de novo synthesis.

Select the most definitive test to differentiate between endogenous destruction and exogenous ingestion.

Measurement of serum thyroglobulin.

Thyroglobulin is co-secreted with endogenous thyroid hormone. It is elevated in destructive thyroiditis but suppressed in exogenous hormone use.

Anahtar Kavram

Differentiating causes of thyrotoxicosis with low radioactive iodine uptake (RAIU) using serum thyroglobulin levels.

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