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Difficulty: EasyEndocrinology and Metabolism

A 42-year-old woman comes to the clinic due to a 6-month history of worsening fatigue, generalized weakness, and cold intolerance. She also reports that she has not had a menstrual period for the past 8 months. She has no significant past medical history and takes no medications. Her temperature is 36.1°C (97.0°F), blood pressure is 95/60 mm Hg, and pulse is 56/min. Physical examination reveals dry skin and a delayed relaxation phase of the deep tendon reflexes. Laboratory studies show:

Laboratory testResultReference range
Thyroid-stimulating hormone (TSH)1.5 µU/mL0.4-4.0 µU/mL
Free thyroxine (T4)0.4 ng/dL0.9-1.7 ng/dL
Pregnancy testNegative-

Which of the following is the most likely diagnosis?

  1. Pituitary adenomaAnswer
  2. B
    Hashimoto thyroiditis
  3. C
    Euthyroid sick syndrome
  4. D
    Iodine deficiency
  5. E
    Graves disease

Answer

Pituitary adenoma
The correct answer identifies that the patient has central hypothyroidism (low free T4 with inappropriately normal TSH). When combined with amenorrhea, this suggests a broader pituitary dysfunction, most commonly caused by a pituitary adenoma compressing the normal pituitary tissue.

Step-by-Step Solution

1
Evaluate the patient's clinical presentation.
The patient has classic symptoms of hypothyroidism (fatigue, cold intolerance, dry skin, delayed deep tendon reflexes) and amenorrhea.
Identifying the clinical syndrome guides the interpretation of the laboratory results.
2
Interpret the thyroid function tests.
Free T4 is low, confirming hypothyroidism. TSH is within normal limits.
Normal or low TSH in the setting of low free T4 indicates a failure of the pituitary to respond to low thyroid hormone levels, pointing to central (secondary) hypothyroidism.
3
Synthesize the findings to determine the etiology.
Central hypothyroidism combined with amenorrhea (likely hypogonadotropic hypogonadism) suggests a mass lesion in the pituitary gland, such as a pituitary adenoma.
A pituitary mass can compress functioning normal tissue, leading to multiple pituitary hormone deficiencies (panhypopituitarism).

Key Concept

Central (secondary) hypothyroidism is characterized by a low free T4 and an inappropriately normal or low TSH level, often caused by a pituitary mass such as an adenoma.
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Topics

Endocrinology and Metabolism

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