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Difficulty: EasyEndocrinology and Metabolism

A 5252-year-old man presents for a routine follow-up appointment due to progressive fatigue, a 4-kg4\text{-kg} (8.8-lb8.8\text{-lb}) weight gain, and an inability to tolerate cold environments over the past 55 months. Three years ago, he underwent surgical resection of a large pituitary adenoma. His temperature is 36.4∘C36.4^\circ\text{C} (97.5∘F97.5^\circ\text{F}), blood pressure is 115/75 mmHg115/75\text{ mmHg}, and pulse is 56/min56/\text{min}. Physical examination reveals dry skin and delayed relaxation of the deep tendon reflexes. Laboratory testing shows:

Serum free T4\text{T}_4: 0.4 ng/dL0.4\text{ ng/dL} (normal: 0.9−1.7 ng/dL0.9-1.7\text{ ng/dL})
Serum TSH\text{TSH}: 1.1 μU/mL1.1\text{ }\mu\text{U/mL} (normal: 0.5−5.0 μU/mL0.5-5.0\text{ }\mu\text{U/mL})

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
    Subacute thyroiditis

Answer

Central hypothyroidism
This patient exhibits classic signs and symptoms of hypothyroidism. In a normally functioning hypothalamic-pituitary-thyroid axis, a low free T4\text{T}_4 level would stimulate the anterior pituitary to secrete high levels of TSH\text{TSH}. This patient's normal TSH\text{TSH} level is inappropriate for his low T4\text{T}_4, indicating a failure of the pituitary gland to respond. Given his history of pituitary adenoma resection, this points directly to central (secondary) hypothyroidism.

Step-by-Step Solution

1
Identify the patient's clinical syndrome.
The patient has signs and symptoms of hypothyroidism (fatigue, weight gain, cold intolerance, delayed reflexes) along with a history of pituitary adenoma resection.
To establish the primary clinical problem and potential underlying causes.
2
Analyze the thyroid function tests.
The free T4 level is abnormally low, confirming the hypothyroid state. However, the TSH level is within the normal range.
To classify the hypothyroidism based on the hypothalamic-pituitary-thyroid axis feedback loop.
3
Synthesize findings to determine the etiology.
A low free T4 with an inappropriately normal TSH in a patient with previous pituitary surgery confirms a diagnosis of central hypothyroidism.
To arrive at the final diagnosis.

Key Concept

Interpretation of thyroid function tests to differentiate primary from central hypothyroidism.
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Topics

Endocrinology and Metabolism

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