TSI may refer to: Science, technology and engineering Technology Schools Initiative Thyroid stimulating immunoglobulin Time-Slot Interchange, communications network switches Total solar irradiance received at top of atmosphere Triple sugar iron test or TSI slant, of a microorganism's ability to ferment sugars Trophic state index of plant nutrients in waterbodies Turbo fuel stratified injection, a VW engine trademark TSI or TFSI Standards, and standards organisations Technical Specifications for Interoperability within European railways Trading Standards Institute, a British professional association Trustworthy Software Initiative, UK Turkish Standards Institution, a public standards organization in Turkey Companies TekSavvy Solutions Inc., Canadian telecommunications company Telesensory Systems Inc., maker of products for the blind Town Sports International, an operator of fitness facilities Organisations The Seasteading Institute, for creating dwellings on seaborne platforms in international waters Three Seas Initiative, European intergovernmental group Media and entertainment Televisione svizzera di lingua italiana, Swiss Italian-language TV Other uses Texas Success Initiative, a guide for developmental education to ensure college readiness in the state of Texas Time in Turkey, generally abbreviated with its usage in Turkish, ''Türkiye Saati İle () Tissue saturation index in medical near-infrared spectroscopy True strength index of financial markets Trauma symptom inventory, a psychological assessment instrument