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Difficulty: EasyGeneral Internal Medicine

A 42-year-old woman presents to the clinic with a 5-month history of progressive fatigue, a 6-kg (13.2-lb) weight gain, and worsening cold intolerance. She also notes that her menstrual cycles have become irregular, occurring every 45 to 60 days. She takes no medications. Her blood pressure is 108/68 mm Hg, and her pulse is 56/min. Physical examination reveals dry skin and delayed relaxation of the deep tendon reflexes. Visual fields are full to confrontation.

Laboratory studies show:
Thyroid-stimulating hormone (TSH): 1.2 mU/L1.2 \text{ mU/L} (Reference: 0.4−4.0 mU/L0.4-4.0 \text{ mU/L})
Free thyroxine (T4): 0.4 ng/dL0.4 \text{ ng/dL} (Reference: 0.9−1.7 ng/dL0.9-1.7 \text{ ng/dL})

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

  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
    Radioactive iodine uptake scan

Answer

Magnetic resonance imaging of the pituitary gland
The correct answer is magnetic resonance imaging of the pituitary gland. This patient presents with classic signs and symptoms of hypothyroidism, including fatigue, weight gain, cold intolerance, bradycardia, and delayed deep tendon reflexes. Her menstrual irregularities (oligomenorrhea) also suggest potential involvement of other pituitary axes. Crucially, her laboratory profile reveals a low free T4 combined with a TSH level that is within the normal reference range. An intact hypothalamic-pituitary-thyroid axis would respond to a low free T4 by significantly increasing TSH production (primary hypothyroidism). When the TSH is inappropriately normal or low in the presence of low free T4, it indicates central (secondary or tertiary) hypothyroidism. The most appropriate next step in evaluation is an MRI of the pituitary gland to check for sellar masses, such as a nonfunctioning pituitary adenoma, which could be compressing normal pituitary tissue. Evaluation of other anterior pituitary hormones (e.g., ACTH, cortisol, prolactin) is also mandatory.

Step-by-Step Solution

1
Interpret the thyroid function tests.
The free T4 level is markedly low, but the TSH level is within the normal reference range.
Accurate interpretation of the TSH relative to the free T4 determines whether the hypothyroidism originates from the thyroid gland (primary) or the pituitary/hypothalamus (central).
2
Identify the pattern of central hypothyroidism.
An inappropriately normal or low TSH in the setting of low free T4 indicates failure of the anterior pituitary to respond to low circulating thyroid hormone levels.
Normal physiological negative feedback should cause TSH to be elevated when T4 is low. The absence of this elevation confirms a central defect.
3
Determine the next appropriate step in diagnostic evaluation.
Imaging of the brain/pituitary is required.
Central hypothyroidism requires an MRI of the pituitary to identify potential structural causes of pituitary failure, such as a pituitary adenoma or empty sella syndrome.

Key Concept

Central (secondary) hypothyroidism is characterized by a low free T4 and an inappropriately normal or low TSH; it requires magnetic resonance imaging of the pituitary gland to evaluate for a central structural lesion.
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Topics

General Internal Medicine

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