A 48-year-old woman presents to the outpatient clinic with a 5-month history of worsening … — USMLE Step 2 CK

A 48-year-old woman presents to the outpatient clinic with a 5-month history of worsening fatigue, unintentional weight gain, and cold intolerance. She also reports bilateral milky nipple discharge and absence of menses for the past 6 months. She has no significant past medical history and takes no medications. Vital signs show a temperature of $36.2^{\circ}\text{C}$ ( $97.2^{\circ}\text{F}$ ), blood pressure of $105/65\text{ mmHg}$ , and pulse of $54\text{/min}$ . Physical examination reveals dry skin and a delayed relaxation phase of the deep tendon reflexes. Visual field testing is unremarkable. Laboratory results are as follows:

Laboratory Test	Result	Reference Range
Thyroid-stimulating hormone (TSH)	$2.1\text{ \mu U/mL}$	$0.5 - 5.0\text{ \mu U/mL}$
Free thyroxine (T4)	$0.4\text{ ng/dL}$	$0.9 - 1.7\text{ ng/dL}$

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

Magnetic resonance imaging of the brainAnswer
B
Initiation of levothyroxine therapy
C
Measurement of anti-thyroid peroxidase antibodies
D
Reassurance and repeat thyroid function tests in 3 months
E
Ultrasound of the thyroid gland

Answer

Magnetic resonance imaging of the brain

This patient presents with classic signs and symptoms of hypothyroidism (fatigue, weight gain, cold intolerance, bradycardia, delayed deep tendon reflexes) and laboratory findings demonstrating a low free T4 level with an inappropriately normal TSH level. This biochemical pattern is the hallmark of secondary (central) hypothyroidism. The concurrent presence of bilateral galactorrhea (milky nipple discharge) and amenorrhea strongly suggests a structural pituitary lesion. A pituitary mass (e.g., a non-functioning macroadenoma) can compress the pituitary stalk, disrupting the normal dopaminergic inhibition of prolactin secretion (leading to hyperprolactinemia and galactorrhea/amenorrhea) while simultaneously compressing thyrotrophs, leading to central hypothyroidism. The most appropriate next step in management is magnetic resonance imaging of the brain to visualize the pituitary gland and hypothalamus.

Step-by-Step Solution

Interpret the thyroid function tests.

The patient has a low free T4 with an inappropriately normal TSH.

To distinguish between primary hypothyroidism (high TSH, low T4) and secondary/central hypothyroidism (normal/low TSH, low T4).

Correlate laboratory findings with the clinical history.

The patient has central hypothyroidism combined with galactorrhea and amenorrhea.

To identify a unifying anatomical diagnosis, which strongly points to a lesion in the sellar region (e.g., a pituitary adenoma causing mass effect on the stalk and compressing normal anterior pituitary tissue).

Determine the necessary diagnostic evaluation.

Brain imaging is indicated.

To visualize the pituitary gland and hypothalamus, confirming the presence, size, and extent of the suspected mass.

Key Concept

Secondary (central) hypothyroidism is characterized by a low free T4 level in the setting of an inappropriately normal or low TSH level. It typically requires an MRI of the brain/pituitary to identify the underlying structural lesion, especially when accompanied by other pituitary hormone deficiencies or hyperprolactinemia.

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