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Difficulty: MediumEndocrinology and Metabolism

A 42-year-old man comes to the physician due to a 3-month history of palpitations, heat intolerance, and increased sweating. He has lost 5 kg (11 lb) during this period despite a normal appetite. He also mentions experiencing mild, dull headaches and occasional difficulty seeing vehicles in his peripheral vision while driving. Vital signs show a heart rate of 110/min and a blood pressure of 140/85 mm Hg. Physical examination reveals a fine hand tremor, hyperreflexia, and a diffusely enlarged, non-tender thyroid gland without palpable nodules. Extraocular movements are intact, and there is no proptosis.

Laboratory studies show:
Thyroid-stimulating hormone (TSH): 7.2 µU/mL (Normal: 0.4–4.0 µU/mL)
Free thyroxine (T4): 2.8 ng/dL (Normal: 0.9–1.7 ng/dL)

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

  1. A
    Radioactive iodine uptake and scan
  2. B
    Measurement of thyroid-stimulating immunoglobulins
  3. Magnetic resonance imaging of the pituitary glandAnswer
  4. D
    Measurement of thyroid peroxidase antibodies
  5. E
    Ultrasonography of the thyroid gland

Answer

Magnetic resonance imaging of the pituitary gland
The patient's clinical presentation is classic for thyrotoxicosis, but his laboratory results show an elevated free T4 paired with an inappropriately elevated TSH. In normal physiology, a high free T4 should suppress TSH production via negative feedback. The presence of an elevated TSH alongside a high free T4 indicates a secondary (central) cause of hyperthyroidism, most commonly a TSH-secreting pituitary adenoma. Furthermore, the patient's associated symptoms of headaches and peripheral vision loss (bitemporal hemianopsia) strongly suggest a mass lesion in the sella turcica compressing the optic chiasm. Therefore, the most appropriate next step in diagnosis is magnetic resonance imaging of the pituitary gland.

Step-by-Step Solution

1
Evaluate the patient's clinical presentation.
The patient exhibits classic symptoms of thyrotoxicosis (weight loss, tremor, heat intolerance, goiter) combined with headaches and bitemporal hemianopsia (peripheral vision loss).
To establish the primary clinical syndrome and identify any localizing signs.
2
Interpret the thyroid function tests.
The free T4 is elevated, but the TSH is inappropriately elevated rather than suppressed.
To differentiate primary from central (secondary) thyroid disorders based on the negative feedback loop.
3
Synthesize the clinical and laboratory findings to determine the underlying etiology.
The combination of central hyperthyroidism and compressive symptoms (headaches, visual field defects) strongly suggests a TSH-secreting pituitary macroadenoma.
To guide the selection of the next appropriate diagnostic step.
4
Select the most appropriate diagnostic imaging.
Magnetic resonance imaging of the pituitary gland is required.
To confirm the presence, size, and extent of the suspected pituitary mass.

Key Concept

Central hyperthyroidism (such as a TSH-secreting pituitary adenoma) presents with thyrotoxicosis and an inappropriately normal or elevated TSH level, requiring a pituitary MRI for definitive diagnosis.
Estimated Time:1m 0s
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Topics

Endocrinology and Metabolism

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