A nurse is caring for a client who has a prescription for levothyroxine. Which of the following laboratory tests should the nurse monitor?

A. Prothrombin time. Levothyroxine does not directly affect coagulation parameters. However, it can enhance the effects of anticoagulants, potentially leading to an increased risk of bleeding, but prothrombin time is not routinely monitored for levothyroxine therapy. B. Arterial blood gases. Levothyroxine does not typically alter arterial blood gas levels. While severe hypothyroidism can lead to respiratory complications, routine ABG monitoring is not required for clients taking this medication. C. Blood urea nitrogen. BUN levels assess kidney function but are not directly influenced by levothyroxine. Although hypothyroidism can cause mild alterations in kidney function, BUN is not the primary test for monitoring levothyroxine therapy. D. Thyroid stimulating hormone. TSH levels are regularly monitored to evaluate the effectiveness of levothyroxine therapy. Since levothyroxine replaces thyroid hormone, TSH levels should decrease as the thyroid hormone levels normalize. Adjustments in dosing are based on TSH results.