This study is a 2-arm, double blinded, randomised clinical trial where 40 participants will be assigned 1:1 to insulin treatment alone (control) or insulin treatment and tirzepatide treatment for 32 weeks. The primary objective is to demonstrate that tirzepatide treatment, dose incremented to 15mg QW for 32 weeks adjunctive to insulin treatment can reduce body weight in patients with T1D and overweight or obesity when compared to insulin treatment alone. The secondary objective is to demonstrate that tirzepatide treatment, dose incremented to 15mg QW for 32 weeks can improve glycaemic control (measured by hbA1c), improve time in range, reduce insulin requirements, and reduce the severity of comorbidities in people with obesity and T1D. This trial includes a 6 month follow-up period.

Inclusion Criteria:

• Age 18-70 years at screening
• A clinical diagnosis of T1D for at least 12 months at time of screening
• Body mass index ≥ 27kg/m2
• HbA1c ≤ 10%
• Capable and willing to self-inject tirzepatide once per week
• In women of childbearing potential, a negative pregnancy test and willing to use effective contraception consistently for the duration of the study
• Able and willing to provide written informed consent for study participation
• Able and willing to use Easy Diet Diary
• Able and willing to keep an exercise log
• Willing to share devices data uploads
• Has current glucagon product to treat severe hypoglycaemia
• Has current ketone meters to check ketones

Exclusion Criteria:

• Age <18 years and >70 years
• A clinical diagnosis of diabetes type other than T1D
• HbA1c > 10%
• Use of GLP-1 receptor agonist within 1 month of study screening
• Use of any glucose lowering medications aside from insulin within 1 month of study screening
• History of hypersensitivity to investigational medicinal product or related product
• Obesity that is induced by other endocrine disorders
• Pregnancy or positive pregnancy test at time of screening, or unwilling to use effective contraception consistently for the duration of the study which is defined in Appendix 1
• Active proliferative diabetic retinopathy, maculopathy, or severe no proliferative diabetic retinopathy requiring acute treatment
• Known gastric emptying abnormality
• History of chronic or acute pancreatitis, uncontrolled hypertension, acute cardiovascular condition within 3 months
• No longer than 12 months of insulin treatment
• Not willing to use a NovoPen 6 to record insulin dosing if currently using multiple daily injections
• Insulin pump, CGM or smart phone devices are not compatible for data transfer
• Not willing to share device data
• Current use of any steroidal medication, or planned long-term steroidal treatment (>4 weeks) during the study period
• Serum triglycerides >500 mg/dL
• History of or plans for bariatric surgery during the study period
• eGFR <45 ml/min/1.73 m2
• History of severe hypoglycaemia (within 3 months of trial period)
• History of diabetic ketoacidosis (within 3 months of trial period)
• History of stroke (within 3 months of trial period)
• History of heart failure
• Planned coronary, carotid, or peripheral artery revascularisation
• History of acute or chronic liver disease
• History of allergy to any form of insulin, GLP-1RA or its excipients
• History of malignancy requiring chemotherapy, surgery, or radiation (within 5 years of trial period)
• History of multiple endocrine neoplasia type 2, familial thyroid cancer, or non-familial medullary thyroid cancer
• Presence or history of malignant neoplasms or in situ carcinomas (other than basal or squamous cell skin cancer, low-risk prostate cancer, or in situ carcinomas of the cervix or carcinoma in situ/high grade prostatic intraepithelial neoplasia) within 5 years before screening
• Have a pacemaker, or metal implants
• Participation in other intervention trials during the study period
• Existence of any additional health conditions or medical issues, including significant psychiatric disorders, that render a person unfit for the study at the discretion of the investigators

De-identified individual participant data related to the primary and secondary outcomes may be shared upon reasonable request following publication. Data sharing will be considered for ethically approved research purposes and subject to data use agreements. IPD will not be shared if legal, ethical, or institutional constraints prevent it.