A 55-year-old man is evaluated in the surgical clinic for a palpable neck lump he noticed … — USMLE Step 2 CK

A $55$ -year-old man is evaluated in the surgical clinic for a palpable neck lump he noticed while shaving. He reports increasing fatigue, cold sensitivity, and occasional dizziness upon standing over the past four months. He also mentions a significant decrease in libido during this period.

Vital signs show a blood pressure of $102/64$ mm Hg and a heart rate of $54$ /min. Physical examination reveals delayed relaxation of deep tendon reflexes and a solitary, firm, $2.2$ -cm nodule in the right lobe of the thyroid gland. No cervical lymphadenopathy is present.

Laboratory results are as follows:
Thyroid-stimulating hormone (TSH): $1.8$ $\mu$ U/mL (normal: $0.4$ – $4.0$ $\mu$ U/mL)
Free thyroxine (T4): $0.3$ ng/dL (normal: $0.9$ – $1.7$ ng/dL)

Ultrasound of the neck demonstrates a $2.2$ -cm, solid, isoechoic right thyroid nodule with regular margins. Which of the following is the most appropriate next step in the management of this patient?

Assessment of serum early morning cortisolAnswer
B
Recommendation for daily levothyroxine supplementation
C
Ultrasound-guided fine-needle aspiration biopsy
D
Radioactive iodine uptake and scintigraphy
E
Measurement of anti-thyroid peroxidase antibodies

Answer

The most appropriate next step is to evaluate the hypothalamic-pituitary-adrenal axis via assessment of serum early morning cortisol.

This patient presents with an incidental solid thyroid nodule and symptomatic hypothyroidism (fatigue, cold intolerance, bradycardia). His laboratory results reveal a low free T4 level paired with an inappropriately normal TSH level. This discordant pattern establishes the diagnosis of central (secondary) hypothyroidism. Furthermore, his symptoms of orthostatic dizziness and decreased libido suggest concurrent central adrenal insufficiency and central hypogonadism, pointing toward panhypopituitarism (e.g., due to a nonfunctioning pituitary macroadenoma). In any patient diagnosed with central hypothyroidism, it is absolutely critical to evaluate the hypothalamic-pituitary-adrenal (HPA) axis before initiating any thyroid hormone replacement. Thyroid hormone increases the basal metabolic rate and accelerates the hepatic clearance of glucocorticoids. Administering levothyroxine to a patient with unrecognized and untreated adrenal insufficiency can abruptly decrease available cortisol and precipitate a life-threatening adrenal crisis. Therefore, assessing serum early morning cortisol is the most appropriate next step. Once adrenal insufficiency is ruled out or adequately treated with glucocorticoids, levothyroxine therapy and structural evaluation of the thyroid nodule (such as ultrasound-guided fine-needle aspiration) can safely proceed.

Step-by-Step Solution

Analyze the patient's presenting symptoms and laboratory findings.

The patient has clinical signs of hypothyroidism (fatigue, bradycardia) and hypogonadism (decreased libido). Labs show a low Free T4 (

0.3

ng/dL) with an inappropriately normal TSH (

1.8

\mu

U/mL).

To establish the primary biochemical diagnosis before addressing the structural finding.

Determine the etiology of the thyroid function test pattern.

The combination of low Free T4 and normal TSH indicates central (secondary) hypothyroidism, likely part of broader panhypopituitarism given his other symptoms.

Primary hypothyroidism would present with an elevated TSH, whereas central hypothyroidism presents with a low or inappropriately normal TSH.

Identify the immediate clinical priority before initiating treatment or further workup of the thyroid nodule.

The hypothalamic-pituitary-adrenal (HPA) axis must be evaluated with a serum early morning cortisol level.

Initiating thyroid hormone replacement in a patient with undiagnosed central adrenal insufficiency can precipitate a fatal adrenal crisis.

Key Concept

In patients presenting with central hypothyroidism, evaluation of the hypothalamic-pituitary-adrenal (HPA) axis is required before initiating levothyroxine therapy to prevent precipitating an acute adrenal crisis.

Estimated Time:1m 30s

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