The thyroid uses iodine to secrete hormones that regulate metabolism. By the late 19th century many patients who had their entire thyroid removed developed lethargy, swelling and growth retardation. Kocher carefully observed patients before and after surgery and showed that these problems stemmed from a lack of thyroid hormone. He introduced reliable bleeding control with clamps and strict antisepsis, lowering surgical mortality from more than 20 % to under 1 %. He also proposed partial thyroidectomy that leaves some tissue behind and preserves hormone function. This idea is still applied in operations for Graves' disease and thyroid cancer today. His work laid the foundation of endocrine surgery.

Emil Theodor Kocher, as professor of surgery at the University of Bern, performed more than 7,000 thyroid operations and prospectively followed postoperative outcomes. He coined the term 'operative myxoedema' to describe the constellation of myxoedema, hypothermia and mental decline that appeared after total thyroidectomy, thereby emphasizing the gland's endocrine nature. Surgically he perfected capsular dissection with early ligation of vessels, dramatically reducing bleeding and recurrent laryngeal nerve injury. His reported mortality of about 0.5 % was far below contemporary averages. Epidemiologically he correlated endemic goiter in the Alps with iodine deficiency, paving the way for public-health interventions such as salt iodisation. By integrating surgery, physiology and population data, Kocher anticipated modern translational research.

Between 1872 and 1912 Kocher compiled 7,196 thyroidectomies, correlating extent of resection with postoperative myxoedematous sequelae. Using clamp-and-tie haemostasis, Listerian antisepsis and subcapsular dissection he cut peri-operative mortality from >20 % to <0.5 % and sharply reduced recurrent laryngeal nerve palsy. He separated diffuse from nodular goitre pathologically and tailored subtotal versus enucleation techniques accordingly. Autologous thyroid transplantation that reversed cretinous features provided functional proof of endocrine activity. Epidemiologic fieldwork in Alpine endemic areas supported the iodine-deficiency hypothesis and informed later population iodisation programmes. The capsular ligation and layered closure principles published in his 1902 monograph remain canonical in endocrine surgery training. Kocher's longitudinal data collection presaged evidence-based medicine and influenced practitioners such as the Mayo brothers. His clinical insights also catalysed Kendall's later isolation of thyroxine, exemplifying interdisciplinarity between surgery and biochemistry.