Thyroid function tests are one of the most frequently requested laboratory investigations worldwide and a recognized area of over-testing. While inappropriate thyroid testing was traditionally discussed in terms of clinical relevance and economic burden, its environmental impact was largely overlooked. Each unnecessary test contributes to healthcare-related carbon emissions, through patient travel, blood collection, single-use consumables, reagent production, analyzer operating and waste management. In the context of growing commitments toward sustainable healthcare and net-zero emissions, laboratory medicine must reassess not only what is tested, but also why and how often. This article explores the intersection between clinical appropriateness and environmental sustainability in thyroid test ordering. Evidence-based strategies, including TSH-first and reflex testing algorithms, can substantially reduce unnecessary free hormone and antibody measurements while preserving diagnostic accuracy. Furthermore, contextualized reference intervals, particularly in elderly populations and during pregnancy, may reduce overdiagnosis and avoid repeated testing driven by physiological variations. Laboratory stewardship programs are therefore a pragmatic strategy for improving clinical quality while contributing to more sustainable healthcare delivery.