A nurse is reviewing the medical record of a client who has been on levothyroxine for several months. Which of the following findings indicates a therapeutic response to the medication?

Levothyroxine is synthetic T4 replacement used in hypothyroidism to restore euthyroid state by normalizing metabolic rate, negative feedback on TSH, and improving systemic endocrine function including cardiovascular and neurologic systems. Rationale: A. Increase in weight indicates worsening or uncontrolled hypothyroidism rather than therapeutic response. Effective levothyroxine therapy increases metabolic activity leading to weight loss. Weight gain reflects persistent low thyroid hormone effect and inadequate replacement dose or poor absorption. B. Decrease in level of thyroxine (T4) indicates under-replacement or worsening thyroid hormone deficiency. Therapeutic levothyroxine increases circulating T4 levels. A decline suggests inadequate dosing or malabsorption. This finding contradicts expected biochemical improvement during effective thyroid hormone replacement therapy. C. Decrease in thyroid stimulating hormone (TSH) indicates effective feedback suppression from restored thyroid hormone levels. Levothyroxine therapy normalizes hypothalamic-pituitary-thyroid axis. TSH reduction reflects adequate replacement and restored euthyroid state. This is the most reliable marker of therapeutic response. D. Increase in hours of sleep may improve general well-being but is nonspecific and not a primary biochemical marker of levothyroxine effectiveness. Symptom-based changes are secondary. Sleep pattern improvement does not reliably indicate euthyroid status compared to TSH normalization.