A 34-year-old woman comes to the outpatient clinic due to profound fatigue, weight gain, cold intolerance, and constipation that have progressively worsened over the last 6 months. She also notes that her menstrual cycles, which were previously regular, have been absent for the past 8 months. She takes no medications and has no other medical conditions. Her blood pressure is 98/60 mm Hg and pulse is 54/min. Physical examination demonstrates dry skin, brittle hair, and delayed relaxation phase of the deep tendon reflexes. Visual fields are full to confrontation.
Laboratory results are as follows:
| Laboratory Test | Result | Reference Range |
|---|---|---|
| Thyroid-stimulating hormone (TSH) | ||
| Free thyroxine (T4) |
Which of the following is the most appropriate next step in management?
- Measurement of early morning serum cortisolAnswer
- BMeasurement of anti-thyroid peroxidase antibodies
- CInitiation of levothyroxine therapy
- DThyroid ultrasonography
- EMeasurement of serum reverse triiodothyronine
Answer
Measurement of early morning serum cortisol
The correct answer is the measurement of early morning serum cortisol. This patient presents with classic signs of hypothyroidism (fatigue, weight gain, cold intolerance, delayed deep tendon reflexes), but her laboratory tests reveal a low free T4 with an inappropriately normal TSH. This discordant pattern indicates central (secondary) hypothyroidism rather than primary thyroid gland failure. Furthermore, the presence of secondary amenorrhea strongly points toward a broader diagnosis of panhypopituitarism, possibly caused by a pituitary adenoma or another sellar lesion. When central hypothyroidism is identified, it is imperative to evaluate the hypothalamic-pituitary-adrenal axis before initiating thyroid hormone replacement. Levothyroxine increases the basal metabolic rate and accelerates the hepatic clearance of cortisol. If a patient has coexisting central adrenal insufficiency, starting levothyroxine without concurrent glucocorticoid coverage can rapidly deplete any remaining cortisol reserves and precipitate a life-threatening acute adrenal crisis.
Step-by-Step Solution
Key Concept
Patients diagnosed with central hypothyroidism must be evaluated for concurrent central adrenal insufficiency before initiating levothyroxine therapy to avoid precipitating an acute adrenal crisis.