A 42-year-old man presents to the primary care office for evaluation of profound sluggishness and persistent constipation over the last five months. He mentions that he needs to wear a heavy sweater even when indoors because he constantly feels chilled. His medical background is significant for surgical removal of a large benign pituitary mass three years prior. On physical examination, his heart rate is 58/min, and there is a noticeable delay in the relaxation phase of his Achilles reflexes. Laboratory testing reveals the following:
| Laboratory Test | Result | Reference Range |
|---|---|---|
| Thyroid-stimulating hormone (TSH) | 1.3 mU/L | 0.5 - 5.0 mU/L |
| Free thyroxine (T4) | 0.6 ng/dL | 0.9 - 1.7 ng/dL |
Which of the following best describes the underlying mechanism for this patient's current condition?
- Insufficient secretion of thyrotropin from the anterior pituitaryCevap
- BPrimary failure of thyroid follicular cells to produce hormone
- CDecreased conversion of circulating thyroxine to its active form
- DAntibodies binding and activating the thyroid follicular receptors
- EImpaired transport of thyroid hormone into target tissues
Cevap
Insufficient secretion of thyrotropin from the anterior pituitary
The correct answer accurately identifies the mechanism of central hypothyroidism. This patient's clinical signs (fatigue, cold intolerance, bradycardia, delayed reflexes) and low free T4 confirm hypothyroidism. However, his TSH is not elevated, as would be expected in primary thyroid failure. A low or inappropriately normal TSH in the setting of low free T4, especially in a patient with a history of pituitary mass resection, indicates inadequate production of thyroid-stimulating hormone (thyrotropin) by the anterior pituitary gland.
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Anahtar Kavram
Central hypothyroidism is characterized by a low free T4 level accompanied by a low or inappropriately normal TSH level, often due to pituitary or hypothalamic injury.
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