A 32-year-old woman comes to the clinic reporting profound fatigue, weight loss, and lightheadedness upon standing over the past 3 months. She delivered a healthy infant 4 months ago. Her delivery was complicated by severe uterine atony and hemorrhage, requiring massive fluid resuscitation and the transfusion of 3 units of packed red blood cells. Since the delivery, she has been unable to breastfeed and has not resumed menstruation. Her blood pressure is 92/58 mm Hg and pulse is 95/min. Laboratory studies show:
| Test | Result | Normal Range |
|---|---|---|
| Serum sodium | 131 mEq/L | 136-145 mEq/L |
| Serum potassium | 4.1 mEq/L | 3.5-5.0 mEq/L |
| Thyroid-stimulating hormone (TSH) | 1.1 mU/L | 0.5-5.0 mU/L |
| Free thyroxine (T4) | 0.5 ng/dL | 0.9-2.4 ng/dL |
The physician suspects a thyroid disorder and considers initiating levothyroxine therapy. Which of the following is the most appropriate next step in management prior to initiating this medication?
- Measure early morning serum cortisolAnswer
- BMeasure serum thyroid peroxidase antibodies
- CObtain an MRI of the brain
- DPerform an ultrasound of the thyroid gland
- EMeasure serum prolactin
Answer
Measure early morning serum cortisol
The correct answer is measuring early morning serum cortisol. This patient’s history of severe postpartum hemorrhage followed by failure to lactate, amenorrhea, profound fatigue, and hypotension is the classic clinical presentation of Sheehan syndrome (ischemic necrosis of the pituitary gland). Her laboratory findings of a low free T4 and an inappropriately normal TSH confirm central hypothyroidism. Because the anterior pituitary is damaged, she likely also has central adrenal insufficiency (decreased ACTH leading to decreased cortisol). Thyroid hormone increases the hepatic clearance of cortisol and elevates the overall basal metabolic rate. Therefore, initiating levothyroxine in a patient with untreated adrenal insufficiency can acutely deplete their limited cortisol reserves and precipitate a life-threatening adrenal crisis. The integrity of the hypothalamic-pituitary-adrenal (HPA) axis must always be evaluated—typically starting with an early morning serum cortisol level—before administering thyroid hormone replacement.
Step-by-Step Solution
Key Concept
In patients with suspected panhypopituitarism and central hypothyroidism (e.g., Sheehan syndrome), evaluation for central adrenal insufficiency with a serum cortisol level is strictly required before initiating levothyroxine therapy in order to prevent an acute adrenal crisis.