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Difficulty: EasyEndocrinology and Metabolism

A 55-year-old woman comes to the clinic reporting progressive fatigue, cold intolerance, and a 4-kg (8.8-lb) weight gain over the past six months. She has a history of a nonfunctioning pituitary macroadenoma for which she underwent transsphenoidal resection two years ago. Vital signs show a heart rate of 55/min and blood pressure of 110/70 mm Hg. Physical examination reveals dry skin and delayed relaxation of deep tendon reflexes. Laboratory results are as follows:

TSH: 0.5 µU/mL (Reference: 0.4-4.0 µU/mL)
Free T4: 0.5 ng/dL (Reference: 0.9-1.7 ng/dL)

Which of the following is the most likely diagnosis?

  1. Central hypothyroidismAnswer
  2. B
    Hashimoto thyroiditis
  3. C
    Subclinical hypothyroidism
  4. D
    Euthyroid sick syndrome
  5. E
    Resistance to thyroid hormone

Answer

Central hypothyroidism
The correct answer is central hypothyroidism. The patient has clinical signs of hypothyroidism and a low free T4 level. In a normally functioning hypothalamic-pituitary-thyroid axis, a low free T4 should trigger a significant elevation in TSH due to the loss of negative feedback. This patient's TSH is inappropriately normal, indicating a failure of the pituitary gland to adequately respond to the low thyroid hormone levels. Her history of transsphenoidal resection for a pituitary macroadenoma makes pituitary failure (central hypothyroidism) the most likely diagnosis.

Step-by-Step Solution

1
Evaluate the patient's clinical presentation.
The patient exhibits classic symptoms and signs of hypothyroidism, including fatigue, cold intolerance, weight gain, bradycardia, and delayed relaxation of deep tendon reflexes.
Identifying the clinical syndrome guides the interpretation of the laboratory findings.
2
Interpret the thyroid function tests in the context of her surgical history.
The free T4 is low, confirming hypothyroidism. However, the TSH is inappropriately normal (0.5 µU/mL) rather than elevated. Combined with her history of pituitary resection, this indicates secondary/central failure.
In primary hypothyroidism, loss of negative feedback leads to a high TSH. A low T4 with a normal or low TSH indicates failure at the level of the pituitary or hypothalamus.

Key Concept

Central (secondary) hypothyroidism is characterized by a low free T4 level and an inappropriately normal or low TSH level, often due to pituitary or hypothalamic disease.
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Topics

Endocrinology and Metabolism

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