Inclusion Criteria:
1. Age ≥12.0 and ≤22 years old at time of consent
2. Clinical diagnosis, based on investigator assessment, of type 1 diabetes for at least one year
3. HbA1c \>6.5 - 12%
4. Currently using insulin for at least six months
5. Currently using the Tandem t:slim insulin pump with Control-IQ Technology® for at least two months
6. Using insulin parameters such as carbohydrate ratio and correction factors consistently on their pump in order to dose insulin for meals or corrections
7. Access to internet and willingness to upload data during the study as needed, including data generated prior to the start of the study
8. For females, not currently known to be pregnant or breastfeeding
9. If female and sexually active, must agree to use a form of contraception to prevent pregnancy while a participant in the study. A negative serum or urine pregnancy test will be required for all females of childbearing potential. Participants who become pregnant will be discontinued from the study. Also, participants who during the study develop and express the intention to become pregnant within the timespan of the study will be discontinued.
10. Willingness to switch to lispro (Humalog) or aspart (Novolog) if not using already, prior to the insulin Optimization Phase and for the duration of the study
11. Willingness to use Lyumjev if randomized to the experimental treatment arm
12. Total daily insulin dose (TDD) at least 10 U/day and not more than 100 U/day
13. Willingness not to start any new non-insulin glucose-lowering agent during the course of the trial (including metformin, GLP-1 agonists, pramlintide, DPP-4 inhibitors, biguanides, sulfonylureas and nutraceuticals)
14. Willingness to eat at least 1 g/kg of carbohydrate per day
15. Willingness to check ketones per study protocol using home urine ketone strips
16. An understanding and willingness to follow the protocol and signed informed consent
Exclusion Criteria:
1. History of diabetic ketoacidosis (DKA) in the 6 months prior to enrollment
2. Severe hypoglycemia resulting in seizure or loss of consciousness in the 6 months prior to enrollment
3. Current HbA1c \<6.5 or \>12
4. Pregnancy or intent to become pregnant during the trial
5. Currently being treated for a seizure disorder
6. Planned surgery during study duration
7. Need for treatment with oral steroid
8. Treatment with any non-insulin glucose-lowering agent (including metformin, GLP-1 agonists, pramlintide, DPP-4 inhibitors, SGLT-2 inhibitors, biguanides, sulfonylureas and nutraceuticals)
9. A known medical condition that in the judgment of the investigator might interfere with the completion of the protocol.
