A 42-year-old man comes to the clinic complaining of worsening anxiety, excessive sweating… — USMLE Step 2 CK

A $42$ -year-old man comes to the clinic complaining of worsening anxiety, excessive sweating, and a $5$ -kg ( $11$ -lb) weight loss over the past three months despite an increased appetite. He has no significant past medical history and takes no medications. Vital signs show a blood pressure of $142/82$ mmHg and a heart rate of $110$ /min. Physical examination reveals a restless patient with a fine resting tremor of the hands, warm and diaphoretic skin, and a palpable, diffusely enlarged, non-tender thyroid gland without discrete nodules. No exophthalmos or pretibial myxedema is noted.

Laboratory evaluation reveals the following:

Test	Result	Reference Range
Thyroid-stimulating hormone (TSH)	$2.8$ $\mu$ U/mL	$0.5$ - $5.0$ $\mu$ U/mL
Free thyroxine (T4)	$3.1$ ng/dL	$0.9$ - $1.7$ ng/dL
Total triiodothyronine (T3)	$245$ ng/dL	$80$ - $200$ ng/dL

Based on the clinical and laboratory findings, which of the following is the most appropriate next diagnostic step?

Magnetic resonance imaging of the pituitary glandAnswer
B
Radioactive iodine uptake and scan
C
Measurement of thyroid-stimulating immunoglobulins
D
Ultrasound of the thyroid gland
E
Measurement of serum thyroglobulin antibodies

Answer

Magnetic resonance imaging of the pituitary gland is the correct next step because the patient's elevated thyroid hormones paired with an inappropriately normal TSH indicate a central cause of hyperthyroidism, most likely a TSH-secreting pituitary adenoma.

The correct answer involves recognizing the classic laboratory pattern for secondary (central) hyperthyroidism. In a normal physiologic state, an elevation in free T4 and T3 exerts robust negative feedback on the anterior pituitary, suppressing TSH production to near-zero levels. When a patient presents with clear thyrotoxicosis and elevated peripheral thyroid hormones, a TSH level that is within the normal range (

2.8

µU/mL) is highly abnormal and termed 'inappropriately normal.' This points directly to autonomous TSH production, most commonly from a TSH-secreting pituitary adenoma (TSH-oma). The definitive next step to evaluate for a central structural lesion is magnetic resonance imaging (MRI) of the pituitary gland.

Step-by-Step Solution

Analyze the patient's clinical presentation.

The patient exhibits classic signs of thyrotoxicosis: weight loss, anxiety, diaphoresis, tachycardia, tremor, and a diffuse goiter.

Establishing the clinical state directs the interpretation of the subsequent biochemical data.

Interpret the thyroid function tests within the context of the hypothalamic-pituitary-thyroid axis.

Free T4 and Total T3 are significantly elevated, confirming clinical hyperthyroidism. However, the TSH is within the normal reference range (

2.8

µU/mL).

In primary hyperthyroidism (e.g., Graves' disease), high levels of T4 and T3 exert negative feedback on the pituitary, causing TSH to be profoundly suppressed (usually <

0.05

µU/mL). A normal TSH in the face of elevated T4/T3 is 'inappropriately normal'.

Determine the etiology based on the 'inappropriately normal' TSH pattern.

This pattern indicates secondary (central) hyperthyroidism, most commonly caused by a TSH-secreting pituitary adenoma (TSH-oma).

The pituitary gland is failing to suppress TSH production despite massive negative feedback from circulating thyroid hormones, indicating autonomous TSH production.

Select the appropriate diagnostic modality to confirm the suspected etiology.

An MRI of the pituitary gland is indicated.

MRI is the imaging modality of choice to visualize and delineate the size and extent of a pituitary macroadenoma or microadenoma responsible for the autonomous TSH secretion.

Key Concept

An elevated free T4 level paired with an inappropriately normal or elevated TSH indicates secondary (central) hyperthyroidism. The most common cause is a TSH-secreting pituitary adenoma, which requires an MRI of the pituitary gland for diagnosis.

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