TSI

• 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

• 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

• TekSavvy Solutions Inc., Canadian telecommunications company
• Telesensory Systems Inc., maker of products for the blind
• Town Sports International, an operator of fitness facilities

• The Seasteading Institute, for creating dwellings on seaborne platforms in international waters
• Three Seas Initiative, European intergovernmental group

• Televisione svizzera di lingua italiana, Swiss Italian-language TV

• 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 ({{translation|With Turkey Time}})
• Tissue saturation index in medical near-infrared spectroscopy
• Total system inertia of an electrical grid
• True strength index of financial markets
• Trauma symptom inventory, a psychological assessment instrument

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