A 35-year-old woman is evaluated in the clinic for progressive fatigue, cold intolerance, and a 5-kg (11-lb) weight gain over the past 4 months. She also reports that she has not had a menstrual period for the last 6 months, despite having previously regular 28-day cycles. She is not taking any medications. Her temperature is 36.4°C (97.5°F), blood pressure is 100/60 mm Hg, and pulse is 58/min. Physical examination reveals dry skin and a delayed relaxation phase of the deep tendon reflexes. A urine pregnancy test is negative.
Laboratory results are as follows:
Thyroid-stimulating hormone (TSH): mU/L (Reference: mU/L)
Free thyroxine (T4): ng/dL (Reference: ng/dL)
Which of the following is the most likely diagnosis?
- AEuthyroid sick syndrome
- BExogenous levothyroxine use
- Central hypothyroidismAnswer
- DHashimoto thyroiditis
- ESubacute granulomatous thyroiditis
Answer
Central hypothyroidism
The correct diagnosis is central hypothyroidism. This patient presents with classic clinical features of hypothyroidism, including fatigue, cold intolerance, weight gain, bradycardia, and delayed deep tendon reflexes. However, her laboratory evaluation shows a low free T4 level accompanied by an inappropriately normal TSH level. In primary hypothyroidism (such as Hashimoto thyroiditis), the lack of negative feedback from low circulating T4 would cause the normal pituitary gland to markedly increase TSH secretion. Therefore, a low free T4 with a normal or low TSH indicates a failure of the hypothalamic-pituitary axis to respond. The patient's concurrent secondary amenorrhea further raises clinical suspicion for a structural pituitary lesion, such as a macroadenoma, causing panhypopituitarism and compressing normal pituitary tissue.
Step-by-Step Solution
Key Concept
Central hypothyroidism is diagnosed when there is a low free T4 accompanied by a normal or low TSH level, indicating a failure of the pituitary to properly respond to low circulating thyroid hormone.