Short answer: Tirosint and Synthroid both contain levothyroxine (T4) for hypothyroidism and TSH suppression in certain thyroid cancers.12 Tirosint is a soft-gel capsule approved from age 6; Synthroid is a tablet approved from birth. Both are taken once a day, 30 to 60 minutes before breakfast, and share a boxed warning against use for weight loss. Neither label reports a trial against the other.
| Tirosint | Synthroid | |
|---|---|---|
| Form | Soft-gel capsule, 13 to 200 mcg, swallowed whole1 | Tablet2 |
| Ages | Adults and children 6 and older1 | Adults and children, including newborns2 |
| When | Once a day, empty stomach, 30 to 60 minutes before breakfast1 | Once a day, empty stomach, 30 to 60 minutes before breakfast2 |
| Full adult dose | About 1.6 mcg/kg/day1 | 1.6 mcg/kg/day2 |
| Dose changes | 12.5 to 25 mcg every 4 to 6 weeks1 | 12.5 to 25 mcg every 4 to 6 weeks2 |
| Side effects | Mainly signs of too much thyroid hormone; no percentages1 | Mainly signs of too much thyroid hormone; no percentages2 |
| Boxed warning | Not for obesity or weight loss1 | Not for obesity or weight loss2 |
The hormone, approved uses, timing and warnings are the same in both labels. The differences are the form (soft-gel capsule vs tablet), the capsule strengths starting at 13 mcg, and age: Tirosint is approved from 6 years, Synthroid from birth.12
Both labels say side effects are mainly those of too much thyroid hormone: fast or irregular heartbeat, nervousness, trouble sleeping, tremor, heat intolerance, sweating, weight loss and diarrhea.12 Doses are adjusted by thyroid blood tests, and any switch between products is for your prescriber.
More detail: Tirosint side effects, Synthroid side effects, the Tirosint and Synthroid dosing schedules, the Levothyroxine guide and Levothyroxine vs liothyronine: what is the difference between T4 and T3?.