Evaluation of Levothyroxine Replacement Therapy After Total Thyroidectomy Using a Smart Wristband (SMART-THY)

Total thyroidectomy is commonly performed for both benign and malignant thyroid diseases. Following surgery, lifelong levothyroxine replacement therapy is required to maintain euthyroidism. The initial levothyroxine dose is generally determined according to clinical characteristics such as body weight and is subsequently adjusted based on thyroid function tests, which are routinely performed approximately 4 to 6 weeks after surgery. During this period, some individuals may experience symptoms related to under-replacement or over-replacement before biochemical assessment becomes available.

Thyroid hormones have well-established effects on the cardiovascular system. Alterations in thyroid hormone levels can influence heart rate, cardiac rhythm, and autonomic nervous system activity. Recent advances in wearable technology have enabled continuous, noninvasive monitoring of physiological parameters, including nighttime heart rate and heart rhythm. These measurements may provide early physiological changes associated with inadequate thyroid hormone replacement.

The primary objective of this prospective observational study is to evaluate whether nighttime heart rate and heart rhythm data obtained from a smart wristband can identify under-replacement or over-replacement of levothyroxine before the routine postoperative thyroid function assessment. Smart wristband-derived physiological measurements will be compared with serum thyroid-stimulating hormone (TSH) and free thyroxine (fT4) levels obtained during standard postoperative follow-up.

The study will enroll adults undergoing total thyroidectomy who require postoperative levothyroxine replacement therapy. Participants will wear a smart wristband during the early postoperative period while continuing standard clinical care. No experimental intervention or modification of routine treatment will be performed. The findings may help determine whether wearable device-derived physiological data can serve as an early, noninvasive indicator of inadequate levothyroxine replacement and support more timely dose adjustment after total thyroidectomy.