A 34-year-old woman comes to the clinic reporting a 6-month history of severe fatigue, a 5-kg (11-lb) weight gain, and generalized weakness. She also reports cold intolerance and irregular menstrual periods, with her last menses occurring 4 months ago. She has no significant past medical history and takes no medications. Blood pressure is 100/60 mmHg and pulse is 55/min. Physical examination shows a tired-appearing woman with dry skin. Deep tendon reflexes exhibit a delayed relaxation phase. Laboratory results are as follows:
| Test | Result | Reference Range |
|---|---|---|
| Thyroid-stimulating hormone (TSH) | 1.8 mIU/L | 0.4 - 4.2 mIU/L |
| Free thyroxine (T4) | 0.5 ng/dL | 0.8 - 2.7 ng/dL |
| Human chorionic gonadotropin | Negative | Negative |
Which of the following is the most appropriate next step in the management of this patient?
- Check serum morning cortisol levelAnswer
- BInitiate levothyroxine therapy
- CMeasure thyroid peroxidase antibodies
- DMeasure serum free testosterone and dehydroepiandrosterone sulfate
- EPerform a radioactive iodine uptake scan
Answer
Check serum morning cortisol level
The correct next step is to check a serum morning cortisol level. This patient presents with classic symptoms of hypothyroidism (fatigue, weight gain, cold intolerance, delayed deep tendon reflexes) and has a correspondingly low free T4. However, her TSH is inappropriately normal rather than elevated. A normal or low TSH in the presence of a low free T4 is the biochemical hallmark of secondary (central) hypothyroidism, which is caused by a failure of the pituitary gland or hypothalamus. In any patient with suspected hypopituitarism, it is critical to evaluate for concurrent central adrenal insufficiency (low ACTH leading to low cortisol). Thyroid hormone replacement (levothyroxine) accelerates the basal metabolic rate and increases the hepatic clearance of cortisol. If levothyroxine is administered to a patient with unrecognized adrenal insufficiency, the sudden relative decrease in available cortisol can precipitate a life-threatening adrenal crisis. Therefore, the hypothalamic-pituitary-adrenal axis must be evaluated (usually with a morning serum cortisol level or an ACTH stimulation test) before initiating levothyroxine. If adrenal insufficiency is confirmed, glucocorticoids must be given before or simultaneously with levothyroxine.
Step-by-Step Solution
Key Concept
In patients with central hypothyroidism (low free T4, normal/low TSH), the adrenal axis must be evaluated (e.g., with a morning cortisol level) before initiating levothyroxine to prevent precipitating an adrenal crisis.
Estimated Time:1m 0s