A 28-year-old man comes to the clinic reporting progressive fatigue, weight gain, and sluggishness over the past 8 months. His medical history is notable for childhood acute lymphoblastic leukemia, which was treated with chemotherapy and cranial irradiation at age 6. He takes no medications. His pulse is 56/min and blood pressure is 110/70 mm Hg. Physical examination shows dry skin and delayed relaxation of the deep tendon reflexes. Laboratory studies show:
| Test | Result | Reference Range |
|---|---|---|
| Thyroid-stimulating hormone (TSH) | 1.1 mU/L | 0.5-5.0 mU/L |
| Free thyroxine (T4) | 0.6 ng/dL | 0.9-1.7 ng/dL |
Which of the following is the most likely diagnosis?
- Central hypothyroidismAnswer
- BPrimary hypothyroidism
- CSubclinical hypothyroidism
- DEuthyroid sick syndrome
- EResistance to thyroid hormone
Answer
Central hypothyroidism
The correct answer is central hypothyroidism. The patient's symptoms (fatigue, weight gain, sluggishness) and physical examination findings (bradycardia, delayed deep tendon reflexes, dry skin) are classic for hypothyroidism. The laboratory evaluation confirms a low free T4 level. However, the TSH is within the normal range, which is inappropriately low for the degree of hypothyroxinemia. In an intact hypothalamic-pituitary-thyroid axis, a low free T4 should trigger a significant increase in TSH production due to the loss of negative feedback. The failure to mount an elevated TSH response, especially in a patient with a known risk factor (history of cranial irradiation), strongly indicates a central (pituitary or hypothalamic) etiology.
Step-by-Step Solution
Key Concept
Diagnosis of central hypothyroidism relies on identifying a low free T4 level accompanied by an inappropriately low or normal TSH level.