An elderly hypothyroid patient with chronic gastrointestinal complaints is being treated with triiodothyronine (T3) rather than thyroxine replacement because the former drug has better gastrointestinal absorption. When compared with no medication, triiodothyronine should have which of the following effects on serum levels of TSH, T3, and T4?

While synthetic or natural thyroxine (T4) is preferred for most hypothyroid patients, synthetic triiodothyronine (T3) is preferred in some patients by some doctors because of its better GI absorption (as in this case) or shoer duration of action (as in patients undergoing sho-term suppression of a thyroid nodule or thyroid cancer patients whose thyroid replacement must be interrupted for chemotherapy). The T3 therapy will increase serum T3 levels, but will not increase T4 levels, as T3 is not conveed peripherally to T4. TSH will be decreased by feedback inhibition by T3 (but to a lesser degree than it would be by T4). This decrease in TSH will cause a decrease in T4.