A 34-year-old woman comes to the clinic due to a 6-month history of worsening fatigue, con… — USMLE Step 2 CK

A 34-year-old woman comes to the clinic due to a 6-month history of worsening fatigue, constipation, and a 6-kg (13.2-lb) weight gain. She also reports that her menstrual periods, previously regular, have ceased entirely for the past 5 months. Recently, she noticed a spontaneous, bilateral milky nipple discharge. Her medical history is unremarkable, and she takes no medications. Vital signs are within normal limits except for a heart rate of 54/min. Physical examination reveals delayed relaxation of the deep tendon reflexes. Confrontational visual field testing demonstrates a loss of peripheral vision bilaterally. Laboratory evaluation shows:

Laboratory Test	Result	Reference Range
Thyroid-stimulating hormone (TSH)	2.5 mU/L	0.5-5.0 mU/L
Free thyroxine (T4)	0.4 ng/dL	0.9-1.7 ng/dL
Prolactin	85 ng/mL	< 25 ng/mL
$\beta$ -hCG	Negative	Negative

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

A
Initiation of levothyroxine therapy
B
Measurement of anti-thyroid peroxidase antibodies
C
Pelvic ultrasonography and measurement of serum testosterone
Magnetic resonance imaging of the pituitary glandAnswer
E
Reassurance and repeat thyroid function testing in 6 weeks

Answer

Magnetic resonance imaging of the pituitary gland

The correct action is to obtain magnetic resonance imaging of the pituitary gland. This patient presents with classic signs of hypothyroidism (worsening fatigue, weight gain, constipation, delayed deep tendon reflexes). Her laboratory results reveal a markedly decreased free thyroxine (T4) level but an inappropriately normal thyroid-stimulating hormone (TSH) level. This pattern is characteristic of secondary (central) hypothyroidism, caused by a failure of the pituitary gland or hypothalamus. Her concurrent symptoms of hyperprolactinemia (galactorrhea, secondary amenorrhea) and a classic visual field defect (bitemporal hemianopsia) point directly to a mass lesion in the sella turcica compressing the optic chiasm and the normal pituitary stalk/tissue. The most likely etiology is a nonfunctioning pituitary macroadenoma or a large prolactinoma. Therefore, dedicated neuroimaging of the pituitary is essential to confirm the diagnosis and determine the structural extent of the mass.

Step-by-Step Solution

Interpret the thyroid function tests

The free T4 is severely low, but the TSH remains within normal limits.

This specific pattern indicates secondary (central) hypothyroidism, demonstrating that the pituitary gland is failing to mount an appropriate TSH response to the lack of circulating thyroxine.

Synthesize the clinical presentation

The patient exhibits central hypothyroidism, hyperprolactinemia (causing galactorrhea and amenorrhea), and a bitemporal hemianopsia.

These combined findings localize the pathology precisely to the sellar/parasellar region, indicating compression of both the normal pituitary tissue and the overlying optic chiasm.

Determine the necessary diagnostic action

Magnetic resonance imaging of the pituitary gland must be ordered.

Neuroimaging is required to visualize the presumed pituitary macroadenoma or structural lesion before proceeding with specialized endocrine testing or surgical intervention.

Key Concept

Secondary (central) hypothyroidism presents with low free T4 and an inappropriately normal or low TSH. When accompanied by signs of mass effect (such as bitemporal hemianopsia) or other pituitary hormone deficiencies/excesses, a pituitary MRI is the indicated next step.

Estimated Time:1m 30s

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