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Difficulty: MediumGeneral Internal Medicine

A 36-year-old woman comes to the outpatient clinic reporting a 4-month history of profound fatigue, a 5-kg (11-lb) weight gain, and persistent constipation. She also mentions that her menstrual periods have stopped completely for the past 3 months. She has no prior medical conditions and takes no daily medications.

Her temperature is 36.6°C (97.8°F), blood pressure is 102/64 mmHg, and pulse is 54/min. Physical examination reveals dry, cool skin and a delayed relaxation phase of the deep tendon reflexes. The thyroid gland is nonpalpable and non-tender. During cranial nerve evaluation, the patient has difficulty seeing fingers presented in the lateral aspects of her visual fields bilaterally.

Initial laboratory testing reveals the following:
TSH: 2.1 μ\mu U/mL (Normal: 0.5-5.0 μ\mu U/mL)
Free T4T_4: 0.4 ng/dL (Normal: 0.9-1.7 ng/dL)

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

  1. Magnetic resonance imaging of the brainAnswer
  2. B
    Initiation of oral levothyroxine therapy
  3. C
    Measurement of serum thyroid peroxidase antibodies
  4. D
    Ultrasound of the thyroid gland
  5. E
    Reassurance and repeat thyroid function testing in 6 months

Answer

Magnetic resonance imaging of the brain
The most appropriate next step in management is magnetic resonance imaging of the brain. This patient presents with classic signs of hypothyroidism, including weight gain, severe fatigue, bradycardia, and delayed deep tendon reflexes. However, her laboratory results show a low free T4 level paired with a TSH level that is within the reference range. In a healthy endocrine system, a low free T4 should trigger a strong compensatory elevation in TSH from the anterior pituitary gland. An inappropriately normal or low TSH in the presence of low free T4 confirms a diagnosis of central (secondary) hypothyroidism. Furthermore, the patient has secondary amenorrhea and bitemporal visual field deficits, which strongly suggest a mass lesion in the sella turcica compressing the optic chiasm and disrupting normal pituitary hormone production (e.g., a nonfunctioning pituitary macroadenoma). Therefore, imaging the brain and pituitary gland is the essential next step to confirm the structural etiology.

Step-by-Step Solution

1
Analyze the patient's clinical symptoms and physical examination findings.
The patient exhibits classic signs of systemic hypothyroidism (fatigue, weight gain, bradycardia, delayed reflexes) accompanied by bitemporal visual field deficits and secondary amenorrhea.
Identifying both the endocrine dysfunction and the localized neurologic deficit is crucial for formulating a differential diagnosis.
2
Interpret the thyroid function tests.
The laboratory results demonstrate a low free T4 level paired with a normal TSH level.
A low free T4 with a normal (or low) TSH indicates central (secondary) hypothyroidism, because a healthy pituitary gland should mount an elevated TSH response to low peripheral thyroid hormone levels.
3
Synthesize the clinical and laboratory data to determine the likely etiology.
The combination of central hypothyroidism, amenorrhea, and bitemporal hemianopsia localizes the lesion to the sella turcica (e.g., a pituitary macroadenoma).
A mass compressing the optic chiasm causes bitemporal hemianopsia, while concurrent compression of the anterior pituitary disrupts normal hormone secretion.
4
Select the most appropriate next step in evaluation based on the suspected etiology.
Magnetic resonance imaging of the brain is indicated.
Imaging is required to visualize the size and extent of the suspected pituitary mass before pursuing medical or surgical interventions.

Key Concept

Secondary (central) hypothyroidism presents with a low free T4 and an inappropriately normal or low TSH. When accompanied by signs of mass effect, such as bitemporal hemianopsia or additional pituitary hormone deficiencies, a pituitary tumor should be suspected and evaluated with an MRI of the brain.
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General Internal Medicine

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