A 42-year-old woman comes to the clinic for evaluation of profound fatigue, weakness, and a 5-kg (11-lb) weight gain over the last 4 months. She also reports cold intolerance and that she has not had a menstrual period for the past 6 months. Her past medical history is unremarkable. She takes no medications. Blood pressure is 110/70 mm Hg, pulse is 52/min, and respirations are 12/min. Physical examination shows a puffy face, dry skin, and delayed relaxation of deep tendon reflexes. The thyroid gland is not palpable. Visual field testing reveals decreased peripheral vision bilaterally.
Laboratory studies show:
| Test | Result | Normal Range |
|---|---|---|
| Sodium | 135 mEq/L | 136–145 mEq/L |
| Potassium | 4.0 mEq/L | 3.5–5.0 mEq/L |
| Thyroid-stimulating hormone (TSH) | 1.1 µU/mL | 0.4–4.0 µU/mL |
| Free thyroxine (T4) | 0.5 ng/dL | 0.9–1.7 ng/dL |
Which of the following is the most appropriate next step in the management of this patient?
- Measure serum morning cortisol levelAnswer
- BInitiate levothyroxine therapy
- CMeasure anti-thyroid peroxidase antibodies
- DOrder an ultrasound of the thyroid gland
- EOrder a radioactive iodine uptake scan
Answer
Measure serum morning cortisol level
This patient presents with classic signs and symptoms of hypothyroidism (fatigue, weight gain, cold intolerance, delayed deep tendon reflexes, bradycardia). Laboratory testing confirms a low free T4. In primary hypothyroidism, the lack of negative feedback leads to a significantly elevated TSH. In this patient, however, the TSH is inappropriately normal, which is the hallmark of central (secondary) hypothyroidism. The presence of amenorrhea and bitemporal hemianopsia (decreased peripheral vision bilaterally) strongly points to a compressive pituitary macroadenoma causing panhypopituitarism. Because the anterior pituitary also produces ACTH, patients with central hypothyroidism frequently have coexisting central adrenal insufficiency. The administration of thyroid hormone increases the metabolic clearance of cortisol. If levothyroxine is given to a patient with untreated adrenal insufficiency, it can rapidly precipitate an acute, life-threatening adrenal crisis. Therefore, the most critical next step is to measure the serum morning cortisol level to evaluate the adrenal axis. An MRI of the brain is also indicated to evaluate the pituitary mass.
Step-by-Step Solution
Key Concept
In patients with central hypothyroidism, the adrenal axis must be evaluated (and treated if deficient) prior to initiating levothyroxine therapy to avoid precipitating an acute adrenal crisis.