T4/T3 Therapy in Hypothyroidism

This study will test whether 6 months of combination therapy with levothyroxine (LT4) plus liothyronine (LT3) is superior to LT4 plus placebo in participants with hypothyroidism who have residual symptoms of hypothyroidism with TSH levels within the reference range.

.Participants will * Take LT4+LT3 or LT4+placebo for 6 months * Visit the clinic at the beginning and the end of the study for a blood test and to answer questionnaires. * Have blood tests checked at 6 and 13 weeks and symptom checks by text (weekly to monthly)

Inclusion Criteria: 1. Male or female, aged 18-70 years 2. Hypothyroidism due to either thyroid failure or total thyroidectomy for structural thyroid disease (nodules, benign goiter, or thyroid cancer) for at least 6 months 3. Taking 75-150 mcg per day LT4 and a minimum of 1.2 mcg/kg/day, with stable dose of LT4 for at least 3 months prior to enrollment 4. TSH of 0.5 to 4.0 mU/L within 2 months of enrollment 5. Persistent symptoms for at least 2 months, and a TSQ score of ≥ 5 6. For females of reproductive potential: use of highly effective contraception for at least 1 month prior to screening and agreement to use such a method during study participation and for an additional 4 weeks after the end of study intervention. Exclusion Criteria: 1. Current use of LT3, thyroid extracts, Tirosint liquid or capsules, amiodarone, lithium, methimazole, propylthiouracil, supraphysiologic doses of corticosteroids, immunotherapy, tyrosine kinase inhibitors, interferon, or biotin supplements 2. Diseases of the pituitary or hypothalamus 3. History of thyroid cancer requiring suppression of TSH secretion 4. Unstable cardiac conditions, including uncontrolled hypertension (current blood pressure greater than 160/100), arrhythmia, or angina 5. Uncontrolled psychiatric disorders 6. Pregnancy or lactation, or planned pregnancy within the next 7 months 7. History of food dye allergies 8. GFR \<30 ml/min/1.73 m2 9. Other conditions which, in the opinion of the investigators, would prevent them from participating in the full duration of the study (for example, end-stage cancer)