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Difficulty: EasyEndocrinology and Metabolism

An asymptomatic 6565-year-old woman visits her primary care physician for an annual checkup. She mentions feeling slightly more fatigued recently, though she attributes this to a busy daily routine. Her medical history includes well-controlled essential hypertension. Her temperature is 37.0∘37.0^\circ C, blood pressure is 125/80125/80 mm Hg, and pulse is 7272/min. Physical examination reveals a normal-sized thyroid gland with no palpable nodules. A screening laboratory evaluation demonstrates the following results:

Thyroid-stimulating hormone (TSH): 7.87.8 mU/L (normal: 0.40.4-4.24.2 mU/L)
Free thyroxine (T4): 1.31.3 ng/dL (normal: 0.80.8-2.72.7 ng/dL)

Which of the following is the most likely diagnosis?

  1. Subclinical hypothyroidismAnswer
  2. B
    Primary hypothyroidism
  3. C
    Central hypothyroidism
  4. D
    Euthyroid sick syndrome
  5. E
    Thyroid hormone resistance

Answer

Subclinical hypothyroidism
The correct answer is subclinical hypothyroidism. This condition is defined biochemically as an elevated serum thyroid-stimulating hormone (TSH) level in the presence of a normal serum free thyroxine (T4) level. Patients are often asymptomatic or have mild, non-specific symptoms such as fatigue. In this vignette, the patient's elevated TSH (7.87.8 mU/L) combined with a normal free T4 (1.31.3 ng/dL) perfectly matches this definition.

Step-by-Step Solution

1
Evaluate the patient's symptoms and physical examination findings.
The patient is largely asymptomatic with a normal physical exam, pointing away from severe overt thyroid disease.
To establish the clinical context before interpreting laboratory values.
2
Interpret the thyroid function tests.
The TSH is elevated (7.87.8 mU/L), indicating decreased thyroid function, but the free T4 remains within the normal range (1.31.3 ng/dL).
To determine the specific pattern of thyroid dysfunction.
3
Synthesize clinical and laboratory findings to establish a diagnosis.
The combination of an elevated TSH and a normal free T4 level is the defining criteria for subclinical hypothyroidism.
To definitively diagnose the condition based on standard diagnostic criteria.

Key Concept

Subclinical hypothyroidism is defined biochemically by an elevated thyroid-stimulating hormone (TSH) level in the presence of a normal free thyroxine (T4) level.
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Topics

Endocrinology and Metabolism

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