A 32-year-old nulliparous woman is evaluated in the clinic for the absence of menstruation… — USMLE Step 2 CK

A 32-year-old nulliparous woman is evaluated in the clinic for the absence of menstruation over the past $8$ months. She and her partner have been having unprotected intercourse for the past year without conception. The patient additionally complains of chronic fatigue, feeling cold even in a warm room, and new-onset dull, bilateral morning headaches. Her vital signs include a blood pressure of $110/70 \text{ mm Hg}$ , heart rate of $58/\text{min}$ , and respiratory rate of $12/\text{min}$ . Body mass index is $24 \text{ kg/m}^2$ . Physical examination shows pale and dry skin, and a delayed relaxation phase of the Achilles tendon reflexes. Visual field testing by confrontation is normal. Pelvic examination is unremarkable.

Laboratory results:
Serum beta-hCG: negative
Follicle-stimulating hormone: $2.4 \text{ mIU/mL}$
Luteinizing hormone: $1.1 \text{ mIU/mL}$
Prolactin: $45 \text{ ng/mL}$ (Normal: $4-23 \text{ ng/mL}$ )
Thyroid-stimulating hormone: $1.2 \text{ mIU/L}$ (Normal: $0.5-5.0 \text{ mIU/L}$ )
Free thyroxine (T4): $0.4 \text{ ng/dL}$ (Normal: $0.9-1.7 \text{ ng/dL}$ )

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

Magnetic resonance imaging of the pituitaryAnswer
B
Measurement of thyroid peroxidase antibodies
C
Assessment of serum total testosterone
D
Endometrial biopsy
E
Initiation of levothyroxine therapy

Answer

Magnetic resonance imaging of the pituitary

The correct answer is magnetic resonance imaging of the pituitary. This patient's presentation of secondary amenorrhea, infertility, and new-onset morning headaches is highly concerning for an intracranial mass. Her laboratory results confirm panhypopituitarism: she has central hypothyroidism (low free T4 with an inappropriately normal TSH) and hypogonadotropic hypogonadism (low gonadotropins leading to hypoestrogenic amenorrhea). The mild elevation in prolactin (

45 \text{ ng/mL}

) is classic for the 'stalk effect.' Large pituitary masses (such as non-functioning macroadenomas) compress the pituitary stalk, disrupting the normal dopaminergic inhibition of lactotrophs by the hypothalamus, leading to mild hyperprolactinemia (usually

<200 \text{ ng/mL}

). MRI of the pituitary is the definitive next step to evaluate for a sellar mass.

Step-by-Step Solution

Interpret the patient's thyroid laboratory results.

Free T4 is low (

0.4 \text{ ng/dL}

), but TSH is inappropriately normal (

1.2 \text{ mIU/L}

). This indicates central (secondary) hypothyroidism rather than primary hypothyroidism.

To identify the level of the hypothalamic-pituitary-thyroid axis defect.

Analyze the reproductive hormone panel.

FSH and LH are inappropriately low-normal despite amenorrhea (indicating central hypogonadism), and prolactin is mildly elevated.

To determine the underlying endocrine etiology of the amenorrhea and infertility.

Synthesize clinical and laboratory findings.

The combination of multiple pituitary hormone deficiencies, mild hyperprolactinemia (stalk effect), and new-onset morning headaches strongly suggests a compressive sellar mass (e.g., a non-functioning pituitary macroadenoma).

To formulate a comprehensive unifying diagnosis.

Determine the next best step in evaluation.

An MRI of the pituitary is the imaging modality of choice to visualize the suspected sellar mass prior to any medical intervention.

To confirm the structural etiology and guide subsequent surgical or medical management.

Key Concept

Panhypopituitarism secondary to a sellar mass can present with amenorrhea and infertility. It is characterized by central hypothyroidism (low free T4 with inappropriately normal or low TSH), hypogonadotropic hypogonadism, and mild hyperprolactinemia due to pituitary stalk compression (the 'stalk effect').

Estimated Time:1m 15s

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