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Difficulty: EasyEndocrinology and Metabolism

A 3838-year-old woman comes to the clinic due to progressive fatigue, weight gain, and cold intolerance over the past 44 months. She also reports that she has not had a menstrual period for the past 55 months. She takes no medications and her past medical history is unremarkable. Her vital signs include a temperature of 36.1∘36.1^\circ C (97.0∘97.0^\circ F), blood pressure of 105/65105/65 mmHg, and a pulse of 5656/min. Physical examination reveals dry skin, facial puffiness, and delayed relaxation of the deep tendon reflexes. A urine pregnancy test is negative.

Laboratory results show:
Thyroid-stimulating hormone (TSH): 0.6μ0.6 \mu U/mL (Normal: 0.4−4.2μ0.4-4.2 \mu U/mL)
Free thyroxine (T4): 0.40.4 ng/dL (Normal: 0.8−2.80.8-2.8 ng/dL)

Which of the following is the most likely diagnosis?

  1. A
    Primary hypothyroidism
  2. B
    Subclinical hypothyroidism
  3. Central hypothyroidismAnswer
  4. D
    Euthyroid sick syndrome
  5. E
    Polycystic ovary syndrome

Answer

Central hypothyroidism
The correct answer is central hypothyroidism. This patient presents with overt clinical signs of hypothyroidism, which is confirmed by her low free T4 level. In a normally functioning hypothalamic-pituitary-thyroid axis, a low free T4 should trigger an increase in TSH production due to the loss of negative feedback (primary hypothyroidism). However, this patient's TSH is inappropriately normal (0.6μ0.6 \mu U/mL). This lack of a robust TSH response points directly to a failure at the level of the pituitary gland or hypothalamus (central hypothyroidism). Her concurrent secondary amenorrhea further supports a broader pituitary dysfunction, such as a pituitary adenoma.

Step-by-Step Solution

1
Evaluate the patient's clinical presentation.
The patient exhibits classic symptoms of hypothyroidism (fatigue, weight gain, cold intolerance, bradycardia, delayed reflexes) along with secondary amenorrhea.
To establish the clinical syndrome before interpreting the laboratory values.
2
Analyze the thyroid function tests.
The free T4 is abnormally low (0.40.4 ng/dL), which biochemically confirms the clinical suspicion of hypothyroidism.
Free T4 determines whether the patient is truly hypothyroid.
3
Assess the TSH response to the low free T4 level.
The TSH is 0.6μ0.6 \mu U/mL, which is inappropriately normal (not elevated) in the setting of a low free T4.
The TSH level differentiates between primary (thyroid gland) and central (pituitary or hypothalamus) causes of hypothyroidism.

Key Concept

Central hypothyroidism is diagnosed when a low free T4 is accompanied by an inappropriately normal or low TSH.
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Topics

Endocrinology and Metabolism

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