A 42-year-old woman presents to her primary care physician reporting progressively worseni… — USMLE Step 2 CK

A 42-year-old woman presents to her primary care physician reporting progressively worsening fatigue and cold intolerance over the past six months. She also notes that her menstrual periods, which were previously regular, ceased entirely eight months ago. Over the last several weeks, she has been experiencing intermittent, dull headaches. She takes no medications and has no significant past medical history.

Physical examination reveals a blood pressure of $110/70$ mmHg and a pulse of $56$ /min. The thyroid gland is not palpable. Neurologic examination demonstrates delayed relaxation of the deep tendon reflexes. Confrontational visual field testing reveals a loss of peripheral vision in both temporal fields.

Laboratory studies show the following:

Laboratory Parameter	Patient Value	Reference Range
Prolactin	$75$ ng/mL	$4 - 23$ ng/mL
Thyroid-stimulating hormone (TSH)	$2.1$ $\mu$ U/mL	$0.4 - 4.0$ $\mu$ U/mL
Free thyroxine (T4)	$0.5$ ng/dL	$0.9 - 1.7$ ng/dL
Urine pregnancy test	Negative	Negative

Which of the following is the most likely underlying mechanism for this patient's clinical and laboratory findings?

Disruption of dopaminergic pathways by a suprasellar massAnswer
B
Primary autoimmune failure of the thyroid gland with secondary thyrotroph hyperplasia
C
Autonomous hypersecretion of prolactin by a functioning pituitary macroadenoma
D
Postpartum ischemic necrosis of the anterior pituitary gland
E
Ectopic production of thyroid-stimulating hormone by an occult malignancy

Answer

The presentation is caused by disruption of dopaminergic pathways by a suprasellar mass (pituitary stalk effect).

The correct answer identifies the 'stalk effect' caused by a non-functioning pituitary macroadenoma. This patient presents with mass effect symptoms (headache, bitemporal hemianopsia) indicating a large sellar lesion. The lesion compresses normal thyrotrophs, causing central hypothyroidism (low Free T4, normal TSH). Simultaneously, it compresses the pituitary stalk (infundibulum). Because prolactin secretion is uniquely regulated by tonic inhibition via hypothalamic dopamine, blocking the stalk prevents dopamine delivery, resulting in a mild elevation of prolactin (usually <

100

ng/mL) and subsequent amenorrhea.

Step-by-Step Solution

Interpret the thyroid function tests.

The patient has a low Free T4 (

0.5

ng/dL) with an inappropriately normal TSH (

2.1

\mu

U/mL).

This specific laboratory pattern indicates central (secondary) hypothyroidism, pointing to a pituitary or hypothalamic origin rather than primary thyroid pathology.

Analyze the structural and neurological findings.

The patient has new-onset headaches and bitemporal hemianopsia.

These findings are classic for a mass lesion in the sella turcica compressing the optic chiasm, such as a pituitary macroadenoma.

Evaluate the prolactin level in the context of the mass lesion.

The prolactin is mildly elevated at

75

ng/mL.

A prolactinoma large enough to cause visual field deficits would typically produce prolactin levels >

200

ng/mL. A mild elevation suggests the 'stalk effect', where a non-functioning mass compresses the infundibulum, blocking the delivery of inhibitory dopamine from the hypothalamus to the lactotrophs.

Key Concept

Non-functioning pituitary macroadenomas can cause central hypopituitarism (via tissue compression) and mild hyperprolactinemia (via pituitary stalk compression, known as the 'stalk effect').

Question

Answer

Step-by-Step Solution

Key Concept

Common Mistakes

Topics

More questions from this topic