Seeking medical advice for the absence of her menstrual periods over the past eight months… — USMLE Step 2 CK

Seeking medical advice for the absence of her menstrual periods over the past eight months, an unmarried twenty-six-year-old nulligravida arrives at the outpatient clinic. Her prior menstrual cycles occurred regularly every thirty days. She reports feeling profoundly fatigued and has noticed a $5$ -kg ( $11$ -lb) weight gain over the past year despite maintaining her usual diet. Recently, she began experiencing intermittent morning headaches.

Vital signs are blood pressure $108/70$ mm Hg, pulse $56$ /min, and BMI $27$ kg/m $^2$ . Physical examination shows delayed relaxation of the deep tendon reflexes and bilateral galactorrhea upon breast expression. There is no facial hirsutism or acne. A point-of-care urine pregnancy test is negative.

Laboratory studies reveal the following:

Laboratory Test	Result	Reference Range
Thyrotropin (TSH)	$0.2$ $\mu$ U/mL	$0.4-4.0$ $\mu$ U/mL
Free tetraiodothyronine (T4)	$0.5$ ng/dL	$0.9-2.4$ ng/dL
Prolactin	$92$ ng/mL	$< 25$ ng/mL
Follicle-stimulating hormone	$3$ mIU/mL	$5-20$ mIU/mL (follicular)

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

Cranial magnetic resonance imagingAnswer
B
Endometrial tissue sampling
C
Prescription of levothyroxine monotherapy
D
Measurement of serum total testosterone
E
Reassurance and repeat pregnancy testing in four weeks

Answer

Cranial magnetic resonance imaging is the most appropriate next step to evaluate for a sellar mass causing pituitary stalk compression.

The correct action is to perform cranial magnetic resonance imaging. This patient presents with secondary amenorrhea, galactorrhea, and clinical features of hypothyroidism (bradycardia, weight gain, delayed reflexes, fatigue). Crucially, her laboratory panel reveals central hypothyroidism (low free T4 with an inappropriately low TSH) and hypogonadotropic hypogonadism (low/normal FSH despite amenorrhea), coupled with a mild elevation in prolactin (

92

ng/mL). This specific constellation is highly characteristic of the 'stalk effect' caused by a sellar or suprasellar mass (such as a nonfunctioning pituitary adenoma or craniopharyngioma). The mass compresses the pituitary stalk, disrupting the downward transport of dopamine from the hypothalamus. Because dopamine normally inhibits prolactin release, this disruption leads to hyperprolactinemia (typically

< 150

ng/mL, whereas true prolactinomas often secrete

> 200

ng/mL). The physical compression of the anterior pituitary simultaneously impairs the release of TSH and gonadotropins (FSH/LH), resulting in hypothyroidism and amenorrhea. An MRI of the brain is the mandatory next step to visualize the mass and determine the extent of compression before any hormonal replacement or surgical intervention is attempted.

Step-by-Step Solution

Evaluate the initial presentation and primary workup.

The patient has secondary amenorrhea, galactorrhea, and symptoms of hypothyroidism, with a negative urine beta-hCG test excluding pregnancy.

Pregnancy is the most common cause of secondary amenorrhea and must always be ruled out first.

Interpret the laboratory findings.

The patient has low free T4 with inappropriately low TSH (central hypothyroidism), low/normal FSH (hypogonadotropic hypogonadism), and mild hyperprolactinemia.

Differentiating primary from central endocrine failure dictates the anatomical location of the pathology.

Deduce the underlying pathophysiology.

A mass lesion is likely compressing the pituitary stalk, blocking inhibitory dopamine from reaching lactotrophs (causing galactorrhea) while physically compressing thyrotrophs and gonadotrophs (causing amenorrhea and hypothyroidism).

This specific combination of deficiencies and mild prolactin elevation is the classic hallmark of the 'stalk effect'.

Select the appropriate diagnostic action.

Cranial magnetic resonance imaging is selected.

Imaging is required to identify the size, exact location, and nature of the suspected mass before medical or surgical intervention.

Key Concept

Secondary amenorrhea with central hypothyroidism and mild hyperprolactinemia strongly suggests a pituitary or suprasellar mass causing stalk compression, requiring MRI of the brain for definitive diagnosis.

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