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Difficulty: EasyInterpreting Laboratory Results

An asymptomatic 55-year-old woman presents for an annual health maintenance visit. She reports feeling well with excellent energy levels, a stable weight, and regular bowel movements. Her only chronic medical condition is mild essential hypertension, which is managed effectively with lisinopril. Her vital signs are within normal limits. On physical examination, her thyroid gland is normal in size, non-tender, and without palpable nodules. The remainder of the physical examination is unremarkable. Routine thyroid function testing reveals the following:

Thyroid-stimulating hormone (TSH): 7.5 µU/mL (Reference: 0.4-4.0 µU/mL)
Free thyroxine (T4): 1.3 ng/dL (Reference: 0.9-1.7 ng/dL)

Which of the following is the most accurate diagnosis based on these laboratory results?

  1. Subclinical hypothyroidismAnswer
  2. B
    Overt primary hypothyroidism
  3. C
    Secondary hypothyroidism
  4. D
    Euthyroid sick syndrome
  5. E
    Thyrotropin-secreting pituitary adenoma

Answer

Subclinical hypothyroidism
The correct answer accurately identifies the patient's laboratory pattern. Subclinical hypothyroidism is a biochemical diagnosis characterized by an elevated serum TSH level alongside a normal serum free T4 level, usually in a patient who is asymptomatic or has very mild, nonspecific symptoms. No immediate treatment is typically required for TSH levels under 10 µU/mL in older, asymptomatic individuals.

Step-by-Step Solution

1
Evaluate the patient's symptoms and physical examination findings.
The patient is completely asymptomatic with a normal physical examination.
To establish the clinical context before interpreting the laboratory values.
2
Analyze the Thyroid-stimulating hormone (TSH) level.
The TSH is elevated at 7.5 µU/mL (normal range 0.4-4.0 µU/mL).
To determine the primary regulatory drive of the thyroid axis.
3
Analyze the free thyroxine (T4) level in the context of the elevated TSH.
The free T4 level is normal at 1.3 ng/dL.
To differentiate between an overt thyroid condition (which would have abnormal free T4) and a subclinical condition.
4
Synthesize the clinical and laboratory findings to make a diagnosis.
An elevated TSH with a normal free T4 in an asymptomatic patient indicates subclinical hypothyroidism.
To arrive at the final, most accurate diagnosis based on established biochemical definitions.

Key Concept

Subclinical hypothyroidism is defined biochemically as an elevated serum TSH concentration in the presence of a normal serum free T4 concentration.
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Interpreting Laboratory Results

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