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Difficulty: MediumEndocrinology and Metabolism

A 55-year-old man presents to the clinic with a 4-month history of worsening fatigue, generalized weakness, and cold intolerance. He also reports a significant decrease in libido and difficulty achieving erections over the same period. The patient occasionally experiences mild, dull headaches but denies any changes in his vision. His blood pressure is 112/72 mm Hg112/72 \text{ mm Hg} and pulse is 56/min56/\text{min}. Physical examination is notable for dry skin, sparse body hair, and delayed relaxation of the deep tendon reflexes. Visual field testing is full to confrontation. Laboratory evaluation is performed, with the following results:

Laboratory TestResultReference Range
Thyroid-stimulating hormone (TSH)1.8 mIU/L1.8 \text{ mIU/L}0.5−5.0 mIU/L0.5 - 5.0 \text{ mIU/L}
Free thyroxine (T4)0.5 ng/dL0.5 \text{ ng/dL}0.9−1.7 ng/dL0.9 - 1.7 \text{ ng/dL}
Total testosterone (8 AM)145 ng/dL145 \text{ ng/dL}300−1000 ng/dL300 - 1000 \text{ ng/dL}

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

  1. Magnetic resonance imaging of the pituitary glandAnswer
  2. B
    Initiation of oral levothyroxine therapy
  3. C
    Measurement of anti-thyroid peroxidase antibodies
  4. D
    Ultrasound of the thyroid gland
  5. E
    Initiation of intramuscular testosterone therapy

Answer

Magnetic resonance imaging of the pituitary gland
The correct answer is magnetic resonance imaging of the pituitary gland. This patient's laboratory results demonstrate a low free T4 with a TSH that is well within the normal range. In primary hypothyroidism, a low free T4 removes negative feedback and triggers a compensatory rise in TSH (typically >10 mIU/L> 10 \text{ mIU/L}). An inappropriately normal or low TSH in the setting of low free T4 is the hallmark of central (secondary) hypothyroidism. Combined with symptoms of hypogonadism, a low morning testosterone level, and headaches, this patient likely has panhypopituitarism secondary to a compressive pituitary mass (such as a nonfunctioning pituitary macroadenoma). The best next step is to obtain a pituitary MRI to evaluate for structural disease. Note that it is also essential to assess the adrenal axis (e.g., via an 8 AM serum cortisol level) before initiating any thyroid hormone replacement.

Step-by-Step Solution

1
Evaluate the thyroid panel results.
The patient has a low free T4 with a TSH that is inappropriately in the normal range, indicating central (secondary) hypothyroidism.
Primary hypothyroidism would cause a compensatory elevation in TSH. A normal or low TSH in the setting of low free T4 localizes the pathology to the pituitary or hypothalamus.
2
Synthesize the laboratory findings with the clinical presentation.
The concurrent hypogonadism (low testosterone, decreased libido) and presence of headaches indicate multi-axis pituitary dysfunction, likely due to a compressive mass.
Panhypopituitarism often presents with sequential loss of anterior pituitary hormones, frequently caused by nonfunctioning pituitary adenomas in adults.
3
Determine the most appropriate diagnostic intervention.
Magnetic resonance imaging (MRI) of the pituitary is required.
Before initiating hormone replacement, the structural cause of the panhypopituitarism must be evaluated via imaging. Additionally, the adrenal axis must be assessed (e.g., 8 AM cortisol) to avoid precipitating an adrenal crisis.

Key Concept

Diagnosis and workup of central hypothyroidism and panhypopituitarism
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Topics

Endocrinology and Metabolism

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