Which hormone is produced synthetically for replacement therapy in hypothyroidism?

• A. Calcitonin
• B. Parathyroid hormone
• C. Insulin
• D. Levothyroxine

Answer: (D)

Replacement therapy for hypothyroidism uses a synthetic thyroid hormone to restore normal metabolism. Levothyroxine provides synthetic thyroxine (T4), which the body's tissues convert into the active T3 as needed. This corrects the hormone deficiency and relieves symptoms such as fatigue and cold intolerance. Dosing is personalized and monitored with thyroid-stimulating hormone (TSH) and free T4 levels to avoid under-treatment or overtreatment, which can cause heart rhythm issues or bone loss over time. Other hormones listed serve different roles: calcitonin is not used to treat hypothyroidism but has other indications like bone health; parathyroid hormone is used to replace missing PTH in hypoparathyroidism; insulin treats diabetes. Therefore, levothyroxine is the standard synthetic hormone for hypothyroidism replacement.