A 45-year-old woman is evaluated in the clinic for an 8-month history of worsening letharg… — USMLE Step 2 CK

A $45$ -year-old woman is evaluated in the clinic for an $8$ -month history of worsening lethargy, unexplained weight gain, and generalized weakness. She also reports that her menstrual periods have been completely absent for the past year. Her blood pressure is $95/60$ mm Hg and her pulse is $55$ /min. Physical examination reveals pale, dry skin and a noticeable delay in the relaxation phase of her deep tendon reflexes.

Laboratory studies reveal the following:

Test	Result	Reference Range
Thyroid-stimulating hormone (TSH)	$1.8$ $\mu$ U/mL	$0.5$ - $5.0$ $\mu$ U/mL
Free thyroxine (T4)	$0.5$ ng/dL	$0.9$ - $1.7$ ng/dL
Follicle-stimulating hormone (FSH)	$3.2$ mIU/mL	$> 30$ mIU/mL (postmenopausal)

Based on these findings, which of the following is the most appropriate next step in management prior to initiating thyroid replacement therapy?

Measurement of early morning serum cortisolAnswer
B
Measurement of anti-thyroid peroxidase antibodies
C
Immediate initiation of levothyroxine monotherapy
D
Ultrasound of the thyroid gland
E
Radioactive iodine uptake scan

Answer

Measurement of early morning serum cortisol

The correct answer is measuring early morning serum cortisol. This patient presents with signs of hypothyroidism and amenorrhea. Her laboratory profile—a low free T4 coupled with an inappropriately normal TSH—is diagnostic of central (secondary) hypothyroidism. Furthermore, her low FSH in the setting of prolonged amenorrhea suggests concurrent hypogonadotropic hypogonadism, pointing broadly toward panhypopituitarism (which could be caused by a pituitary adenoma or other sellar pathology). In patients with panhypopituitarism, central adrenal insufficiency is highly likely. Because exogenous thyroid hormone increases the hepatic clearance of cortisol, initiating levothyroxine therapy without first assessing and, if necessary, treating underlying adrenal insufficiency can rapidly precipitate a potentially fatal adrenal crisis. Therefore, evaluating the adrenal axis with an early morning serum cortisol is the mandatory next step.

Step-by-Step Solution

Interpret the thyroid laboratory panel.

The patient has a low free T4 with an inappropriately normal TSH, indicating central (secondary) hypothyroidism.

In primary hypothyroidism, the low T4 would cause a compensatory elevation in TSH. A normal or low TSH with low T4 confirms a pituitary or hypothalamic failure.

Correlate with additional endocrine findings.

The patient has amenorrhea with an inappropriately low FSH for a postmenopausal-aged woman, indicating concurrent hypogonadotropic hypogonadism.

Multiple pituitary hormone deficiencies suggest panhypopituitarism (e.g., due to a pituitary adenoma).

Identify the required safety protocol for initiating thyroid replacement in panhypopituitarism.

Adrenal axis function must be evaluated (and treated if deficient) prior to starting levothyroxine.

Thyroid hormone increases the metabolic clearance of cortisol. If a patient has concurrent central adrenal insufficiency, starting levothyroxine without glucocorticoid coverage can precipitate a fatal adrenal crisis.

Key Concept

In patients with suspected central hypothyroidism or panhypopituitarism, central adrenal insufficiency must be ruled out (e.g., via early morning serum cortisol) prior to initiating levothyroxine therapy to avoid precipitating an acute adrenal crisis.

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