A 33-year-old woman comes to the clinic reporting the absence of menses for the past 9 mon… — USMLE Step 2 CK

A $33$ -year-old woman comes to the clinic reporting the absence of menses for the past $9$ months. Her menarche occurred at age $12$ , and her menstrual cycles were regular every $28$ days until $9$ months ago. She also reports progressive fatigue, feeling cold constantly, and a $6\text{-kg}$ ( $13\text{-lb}$ ) weight gain over the past year despite no changes in her diet or exercise routine. She occasionally notices a milky white discharge from her breasts. The patient has no headaches, visual field deficits, or abnormal facial hair growth. She takes no medications and uses condoms for contraception. Vital signs show a blood pressure of $110/70\text{ mm Hg}$ , pulse of $58/\text{min}$ , and temperature of $36.5^\circ\text{C}$ ( $97.7^\circ\text{F}$ ). Her body mass index is $26\text{ kg/m}^2$ . The thyroid gland is not palpable. Breast examination reveals bilateral, non-bloody, milky discharge that is easily expressible on palpation. Neurologic examination shows a delayed relaxation phase of the patellar reflexes. Visual fields are full to confrontation. A urine pregnancy test is negative.

Laboratory studies show the following:
Prolactin: $68\text{ ng/mL}$ (Normal: $4$ - $23\text{ ng/mL}$ )
Thyroid-stimulating hormone (TSH): $0.2\text{ } \mu\text{U/mL}$ (Normal: $0.5$ - $5.0\text{ } \mu\text{U/mL}$ )
Free thyroxine (T4): $0.4\text{ ng/dL}$ (Normal: $0.9$ - $2.4\text{ ng/dL}$ )

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

Magnetic resonance imaging of the brainAnswer
B
Initiation of levothyroxine therapy
C
Prescription of combined oral contraceptives
D
Measurement of serum testosterone and dehydroepiandrosterone sulfate
E
Endometrial biopsy

Answer

Magnetic resonance imaging of the brain

The correct answer is magnetic resonance imaging of the brain. This patient's presentation of secondary amenorrhea, fatigue, cold intolerance, and delayed deep tendon reflexes strongly suggests clinical hypothyroidism. The laboratory findings of a low free T4 coupled with an inappropriately low TSH confirm central (secondary) hypothyroidism, indicating pituitary dysfunction. The concurrent mild hyperprolactinemia (galactorrhea) is highly suspicious for a large pituitary mass (such as a non-functioning macroadenoma) causing pituitary stalk compression (the 'stalk effect'). The stalk effect prevents hypothalamic dopamine from reaching the anterior pituitary, thereby removing the normal tonic inhibition of lactotrophs and causing hyperprolactinemia (typically

<100\text{ ng/mL}

). An MRI of the brain is essential to evaluate the pituitary gland for a mass before initiating any hormone replacement.

Step-by-Step Solution

Evaluate the patient's symptoms and physical exam findings.

The patient presents with secondary amenorrhea, galactorrhea, weight gain, fatigue, bradycardia, and delayed deep tendon reflexes.

Identifying the clinical syndrome points toward concurrent hypothyroidism and hyperprolactinemia.

Interpret the endocrine laboratory results.

Free T4 is low, but TSH is inappropriately low/normal. Prolactin is moderately elevated (

68\text{ ng/mL}

A low T4 with a low TSH indicates central (secondary) hypothyroidism. The elevated prolactin, typically

<100\text{ ng/mL}

in this context, suggests pituitary stalk compression rather than a true prolactinoma (which often secretes

>200\text{ ng/mL}

Determine the most appropriate diagnostic intervention based on the suspected etiology.

Select magnetic resonance imaging of the brain/pituitary.

A mass compressing the pituitary stalk disrupts the delivery of hypothalamic dopamine to lactotrophs (causing hyperprolactinemia) and impairs thyrotroph function (causing central hypothyroidism). Imaging is required to visualize the lesion before any treatment.

Key Concept

Secondary amenorrhea with central hypothyroidism (low TSH, low T4) and mild hyperprolactinemia suggests a pituitary macroadenoma causing mass effect (stalk compression); MRI of the brain is the required next step.

Estimated Time:1m 30s

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