A patient with a deiodinase enzyme defect has high T4 levels, yet exhibits hypothyroid symptoms. Why does this condition cause a seemingly paradoxical presentation?

💡 Explanation

The deiodinase enzymes are essential for converting T4 into the more active T3 form. Because the patient's enzyme is defective, T4 accumulates without producing sufficient T3 to stimulate thyroid hormone receptors; therefore, hypothyroidism ensues, rather than hyperthyroidism, despite elevated T4.

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