- Thrombopenia (see also Thrombocytopenia) 287.5
- Thrombophlebitis 451.9
- Thrombosis, thrombotic (marantic) (multiple) (progressive) (septic) (vein) (vessel) 453.9
- Thrombus - see Thrombosis
- Thrush 112.0
- Thumb - see also condition
- Thygeson's superficial punctate keratitis 370.21
- Thymergasia (see also Psychosis, affective) 296.80
- Thymitis 254.8
- Thymoma (benign) (M8580/0) 212.6
- Thymus, thymic (gland) - see condition
- Thyrocele (see also Goiter) 240.9
- Thyroglossal - see also condition
- Thyroid (body) (gland) - see also condition
- Thyroiditis 245.9
- Thyrolingual duct, persistent 759.2
- Thyromegaly 240.9
- Thyrotoxic
- Thyrotoxicosis 242.9
- TIA (transient ischemic attack) 435.9
- Tibia vara 732.4
- Tic 307.20