A 38-year-old woman comes to the office due to a 4-month history of worsening fatigue, gen… — USMLE Step 2 CK

A 38-year-old woman comes to the office due to a 4-month history of worsening fatigue, generalized weakness, and a $5$ -kg ( $11$ -lb) weight gain. She reports that her menstrual periods ceased 6 months ago; previously, her cycles were regular. She also complains of occasional dull, non-throbbing headaches. Her medical history is unremarkable, and she takes no medications. Vital signs show a temperature of $36.4^\circ$ C ( $97.5^\circ$ F), blood pressure of $105/65$ mm Hg, and pulse of $56$ /min. Physical examination reveals delayed relaxation of deep tendon reflexes, and pale, dry skin. There is no goiter. Visual fields are full to confrontation. A urine pregnancy test is negative.

Laboratory results show:

Laboratory Test	Result	Reference Range
TSH	$1.1$ $\mu$ U/mL	$0.5$ – $5.0$ $\mu$ U/mL
Free T4	$0.4$ ng/dL	$0.9$ – $1.7$ ng/dL
Prolactin	$12$ ng/mL	$4$ – $23$ ng/mL

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

Measurement of early morning serum cortisolAnswer
B
Initiation of levothyroxine therapy
C
Measurement of serum testosterone
D
Ultrasound of the thyroid gland
E
Initiation of liothyronine therapy

Answer

Measurement of early morning serum cortisol

The correct answer is measurement of early morning serum cortisol. This patient exhibits classic signs of hypothyroidism (fatigue, weight gain, bradycardia, delayed deep tendon reflexes) accompanied by a low serum free T4 and an inappropriately normal TSH. This laboratory pattern is diagnostic of central (secondary) hypothyroidism, pointing to a hypothalamic or pituitary etiology (supported by her new-onset headaches and amenorrhea). When global pituitary dysfunction is suspected, it is critical to evaluate the hypothalamic-pituitary-adrenal (HPA) axis before treating the thyroid axis. Thyroid hormone replacement increases the basal metabolic rate and accelerates the hepatic metabolism and clearance of cortisol. If a patient has coexisting undiagnosed central adrenal insufficiency, administering levothyroxine can rapidly deplete their remaining cortisol stores and precipitate a life-threatening adrenal crisis. Therefore, cortisol levels must be checked, and if adrenal insufficiency is present, glucocorticoid replacement must be initiated prior to or concurrently with thyroid hormone.

Step-by-Step Solution

Interpret the thyroid function tests in the context of the clinical presentation.

The patient has clinical signs of hypothyroidism (fatigue, bradycardia, delayed reflexes) with a markedly low free T4 level but an inappropriately normal TSH.

This specific laboratory pattern indicates central (secondary) hypothyroidism, as a primary thyroid failure would present with an elevated TSH.

Identify the implications of central hypothyroidism on other endocrine axes.

Central hypothyroidism is frequently caused by a mass (e.g., nonfunctioning pituitary adenoma) or other process disrupting global pituitary function, making concomitant central adrenal insufficiency highly likely.

The presence of headaches and amenorrhea further suggests a broader hypothalamic-pituitary mass effect or multi-axis failure.

Determine the necessary safety check before initiating thyroid hormone replacement.

An early morning serum cortisol must be measured to rule out adrenal insufficiency before starting levothyroxine.

Administering thyroid hormone increases the basal metabolic rate and hepatic clearance of cortisol. In a patient with underlying adrenal insufficiency, this sudden clearance can precipitate a life-threatening adrenal crisis.

Key Concept

In patients with central hypothyroidism (low free T4 with low/normal TSH), concomitant central adrenal insufficiency must be ruled out (via morning cortisol) before initiating levothyroxine to prevent precipitating an adrenal crisis.

Estimated Time:1m 30s

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