Final answer:
A therapeutic effect of methimazole for hyperthyroidism would be indicated by decreased levels of T4, as it signifies the medication is successfully reducing the overproduction of thyroid hormones.
Step-by-step explanation:
When evaluating the therapeutic effect of methimazole in a client with hyperthyroidism, a nurse would look for a decreased T4 level. Methimazole works to decrease the production of thyroid hormones, which in a hyperthyroid state are excessively high and lead to an increased metabolic rate. The correct choice is D. Decreased T4, as thyroid hormones, including thyroxine (T4), have a calorigenic effect and are responsible for increased metabolism and body heat production. When methimazole is effective, the levels of T4 in the blood decrease to within normal range, indicating a lessened activity of the thyroid gland and the restoration of normal metabolic rates.