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Difficulty: EasyEndocrinology and Metabolism

A 48-year-old man comes to the clinic reporting progressive fatigue, decreased libido, and constant cold intolerance over the past six months. He also mentions occasionally bumping into objects on his far right and left sides. Vital signs are within normal limits, though his heart rate is 54/min. Physical examination reveals delayed relaxation of deep tendon reflexes and a visual field deficit in the outer half of both eyes. Laboratory testing shows:

TestResultReference Range
Thyroid-stimulating hormone (TSH)1.2 mU/L0.4 - 4.0 mU/L
Free T40.4 ng/dL0.9 - 1.7 ng/dL

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

  1. Magnetic resonance imaging of the brainAnswer
  2. B
    Measurement of anti-thyroid peroxidase antibodies
  3. C
    Radioactive iodine uptake scan
  4. D
    Ultrasound of the thyroid gland
  5. E
    Initiation of levothyroxine therapy without further imaging

Answer

Magnetic resonance imaging of the brain
This patient presents with signs and symptoms of hypothyroidism (fatigue, cold intolerance, bradycardia, and delayed deep tendon reflexes) combined with decreased libido and bitemporal hemianopsia. The laboratory findings of a low free T4 paired with an inappropriately normal TSH confirm the diagnosis of central (secondary) hypothyroidism. The visual field deficit indicates a mass lesion in the sella turcica that is extending superiorly and compressing the optic chiasm. The most likely etiology is a pituitary macroadenoma (such as a non-functioning adenoma or a prolactinoma). The most appropriate next step in evaluation is an MRI of the brain to characterize the mass before planning surgical or medical intervention.

Step-by-Step Solution

1
Analyze the patient's clinical presentation.
The patient has symptoms of hypothyroidism (fatigue, cold intolerance, bradycardia, delayed reflexes) alongside bitemporal hemianopsia (loss of outer visual fields).
Identifying the clinical syndrome guides the interpretation of the laboratory data.
2
Interpret the thyroid function tests.
Free T4 is low, but TSH is inappropriately normal (1.2 mU/L), which points to a central (pituitary or hypothalamic) etiology rather than a primary thyroid gland failure.
In primary hypothyroidism, a low T4 would trigger a compensatory elevation in TSH. A normal or low TSH with a low T4 is the hallmark of central hypothyroidism.
3
Synthesize findings to determine the next best step.
The combination of central hypothyroidism and optic chiasm compression strongly suggests a large pituitary mass (macroadenoma). Therefore, brain imaging is required.
MRI of the brain is the gold standard for visualizing the sella turcica and evaluating for pituitary tumors.

Key Concept

Patients with a low free T4 and an inappropriately normal or low TSH have central hypothyroidism, which necessitates an MRI of the brain to evaluate for a pituitary mass, especially when accompanied by compressive symptoms like bitemporal hemianopsia.
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Topics

Endocrinology and Metabolism

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