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Difficulty: MediumEndocrinology and Metabolism

A 5555-year-old man comes to the clinic reporting a 66-month history of worsening fatigue, generalized weakness, and constipation. He also notes experiencing frequent dull headaches and has recently been bumping into doorframes while walking. His medical history is unremarkable, and he takes no medications. Vital signs are within normal limits. Physical examination reveals dry, cool skin and delayed relaxation of the deep tendon reflexes. Confrontation visual field testing demonstrates decreased peripheral vision bilaterally. Laboratory results are as follows:

Thyroid-stimulating hormone (TSH): 2.12.1 μ\mu U/mL (Normal: 0.50.5–5.05.0 μ\mu U/mL)
Free thyroxine (T4): 0.50.5 ng/dL (Normal: 0.90.9–2.42.4 ng/dL)

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

  1. Magnetic resonance imaging of the pituitary glandAnswer
  2. B
    Measurement of serum thyroid peroxidase antibodies
  3. C
    Initiation of levothyroxine therapy
  4. D
    Ultrasound of the thyroid gland
  5. E
    Measurement of serum reverse triiodothyronine

Answer

Magnetic resonance imaging of the pituitary gland
This patient presents with clinical signs of hypothyroidism (fatigue, constipation, delayed deep tendon reflexes) and a laboratory profile showing a low free T4 level with an inappropriately normal TSH. This pattern is diagnostic of central (secondary) hypothyroidism, indicating failure at the level of the pituitary or hypothalamus. Furthermore, his headaches and bitemporal hemianopsia (decreased peripheral vision causing him to bump into objects) indicate a mass lesion compressing the optic chiasm. The most likely underlying cause is a pituitary macroadenoma causing both hypopituitarism and mass effect. Therefore, the most appropriate next step is magnetic resonance imaging of the pituitary gland to visualize the sella turcica and the extent of optic chiasm compression.

Step-by-Step Solution

1
Interpret the patient's symptoms and physical examination findings.
The patient exhibits classic symptoms of hypothyroidism (fatigue, constipation, delayed reflexes). Additionally, headaches and bitemporal hemianopsia (bumping into doorframes) suggest a mass lesion compressing the optic chiasm.
Identifying both the endocrine and neurologic deficits narrows the differential diagnosis to a central structural cause.
2
Analyze the thyroid laboratory panel.
The free T4 is significantly low, but the TSH is inappropriately normal rather than elevated.
This pattern distinguishes secondary (central) hypothyroidism from primary thyroid failure, where TSH would be strongly elevated due to lack of negative feedback.
3
Determine the appropriate management based on the combined clinical picture.
The combination of central hypothyroidism and optic chiasm compression points to a pituitary macroadenoma, requiring urgent structural imaging.
Visualizing the sella turcica is critical for confirming the mass, assessing its extent, and planning potential surgical intervention before initiating hormone replacement therapy.

Key Concept

Diagnosis and management of central hypothyroidism secondary to a pituitary macroadenoma.
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Topics

Endocrinology and Metabolism

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