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Difficulty: MediumGeneral Internal Medicine

A 36-year-old woman comes to the office due to severe fatigue, cold intolerance, and constipation that have progressively worsened over the past 5 months. She also reports secondary amenorrhea for the last 6 months. She takes no daily medications. Her blood pressure is 96/60 mm Hg and her pulse is 54/min. Physical examination reveals dry skin, thinning of the lateral eyebrows, and a delayed relaxation phase of the deep tendon reflexes.

Laboratory studies:
- Hemoglobin: 11.011.0 g/dL
- Thyroid-stimulating hormone (TSH): 1.2 μU/mL1.2 \ \mu\text{U/mL} (Normal: 0.50.5–5.0 μU/mL5.0 \ \mu\text{U/mL})
- Free thyroxine (T4): 0.40.4 ng/dL (Normal: 0.90.9–1.71.7 ng/dL)
- Urine pregnancy test: Negative

Which of the following is the most appropriate next step in management?

  1. Measurement of serum early morning cortisolAnswer
  2. B
    Initiation of oral levothyroxine
  3. C
    Measurement of serum thyroid peroxidase antibodies
  4. D
    Performance of a radioactive iodine uptake scan
  5. E
    Observation and repeat thyroid function tests in 3 months

Answer

Measurement of serum early morning cortisol
This patient's clinical presentation (fatigue, cold intolerance, delayed reflexes) and laboratory results (low free T4) confirm hypothyroidism. However, her thyroid-stimulating hormone (TSH) is inappropriately normal. In primary hypothyroidism, the lack of negative feedback from T4 leads to a markedly elevated TSH. The presence of a low free T4 with a low-to-normal TSH indicates central (secondary) hypothyroidism, localizing the defect to the pituitary or hypothalamus. Her concurrent secondary amenorrhea further suggests a broader process affecting multiple axes, such as panhypopituitarism. Before initiating thyroid hormone replacement in any patient with central hypothyroidism, it is critical to evaluate the hypothalamic-pituitary-adrenal axis. Thyroid hormone increases basal metabolic rate and hepatic clearance of cortisol. If concurrent central adrenal insufficiency is present and unrecognized, administering levothyroxine without prior glucocorticoid coverage can precipitate a life-threatening adrenal crisis. Therefore, assessing the adrenal axis with a morning serum cortisol level is the most appropriate next step.

Step-by-Step Solution

1
Analyze the thyroid function tests in the context of the patient's hypothyroid symptoms.
The patient has a low free T4 but a normal TSH, indicating central (secondary) hypothyroidism.
A normal TSH in the setting of low T4 is inappropriate; a properly functioning pituitary would secrete high levels of TSH in response to low T4.
2
Correlate the central hypothyroidism with the patient's secondary amenorrhea.
The combination of multiple central hormone deficiencies strongly suggests panhypopituitarism.
Pituitary pathology (e.g., adenoma, Sheehan syndrome, lymphocytic hypophysitis) typically affects multiple hormone axes simultaneously.
3
Determine the safest sequence for hormone replacement therapy.
The adrenal axis must be evaluated (with a morning cortisol) before initiating thyroid hormone replacement.
Thyroid hormone replacement increases the basal metabolic rate and hepatic clearance of cortisol. In a patient with unrecognized central adrenal insufficiency, this can precipitate a fatal adrenal crisis.

Key Concept

Evaluation sequence in panhypopituitarism and central hypothyroidism

Alternative Method

When evaluating central endocrine failure, remembering the phrase 'steroids before thyroids' ensures that the critical step of checking cortisol and initiating glucocorticoids (if deficient) happens before levothyroxine is given.
Estimated Time:1m 15s
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Topics

General Internal Medicine

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