A 32-year-old woman comes to the clinic for evaluation of progressive fatigue, generalized weakness, and cold intolerance that has worsened over the past 4 months. She also reports that her menstrual periods, which were previously regular, have been absent for the past 6 months. She has no significant past medical history and takes no medications. Vital signs show a blood pressure of 100/65 mm Hg and a pulse of 52/min. Physical examination reveals dry skin and a delayed relaxation phase of the deep tendon reflexes. The thyroid gland is not palpable. Laboratory testing is obtained and reveals the following:
| Laboratory Test | Result | Reference Range |
|---|---|---|
| Thyroid-stimulating hormone (TSH) | 1.1 mU/L | 0.5–5.0 mU/L |
| Free thyroxine (T4) | 0.4 ng/dL | 0.9–1.7 ng/dL |
Which of the following is the most likely underlying cause of this patient's clinical presentation?
- Decreased anterior pituitary secretion of thyrotropinAnswer
- BAutoimmune destruction of the thyroid gland follicles
- CInadequate dietary intake of iodine
- DSevere systemic illness altering thyroid hormone metabolism
- EExcessive androgen production from the ovaries
Answer
The most likely cause of this patient's presentation is a decrease in anterior pituitary secretion of thyrotropin, indicating central hypothyroidism.
The correct answer identifies that the patient has central (secondary) hypothyroidism. The patient presents with classic symptoms of a hypothyroid state, including profound fatigue, cold intolerance, bradycardia, and delayed deep tendon reflexes. Her laboratory results show a markedly low free T4 level. In primary hypothyroidism (where the thyroid gland itself fails), the anterior pituitary would respond to this low T4 by secreting high amounts of TSH. Because this patient's TSH is inappropriately normal, the defect lies in the anterior pituitary gland failing to secrete adequate thyrotropin (TSH). A common cause for this in young women is a pituitary adenoma, which can also disrupt the secretion of gonadotropins or increase prolactin, explaining her 6-month history of amenorrhea.
Step-by-Step Solution
Key Concept
Central (secondary) hypothyroidism is characterized by clinical features of hypothyroidism accompanied by a low free T4 and an inappropriately normal or low TSH level.