Short answer: Armour Thyroid is dried pig thyroid giving 38 mcg of T4 and 9 mcg of T3 per grain (60 mg); its label states it has not been approved by FDA as a new drug.1 Synthroid is FDA-approved levothyroxine (T4 only), dosed in micrograms.2 Both labels carry a boxed warning against use for weight loss. Neither label reports a trial against the other.
| Armour Thyroid | Synthroid | |
|---|---|---|
| What is in it | Dried pig thyroid: 38 mcg T4 and 9 mcg T3 per grain (60 mg)1 | Levothyroxine (T4)2 |
| FDA status | Label states it has not been approved by FDA as a new drug1 | FDA-approved prescription label2 |
| Strengths | 15 mg (1/4 grain) to 300 mg (5 grains)1 | Tablets in micrograms2 |
| Starting dose | Usually 30 mg, up 15 mg every 2 to 3 weeks; 15 mg with long-standing myxedema1 | Full adult dose 1.6 mcg/kg/day; some start lower; up 12.5 to 25 mcg every 4 to 6 weeks2 |
| Usual maintenance | 60 to 120 mg a day1 | Set by thyroid blood tests2 |
| Side effects | Rare apart from 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 |
What is in the tablet. Armour Thyroid contains both T4 and T3 from pig thyroid glands; its label notes T3 is about four times as potent as T4 microgram for microgram.1 Synthroid contains only T4 (levothyroxine).2 The Armour Thyroid label states that it has not been approved by FDA as a new drug; Synthroid has a full FDA-approved label.12
Doses are on different scales (grains or mg vs mcg) and are not swapped one for one without your prescriber. Both labels list mainly signs of too much thyroid hormone as side effects, such as a fast pulse, palpitations, sweating and nervousness.12
More detail: Armour Thyroid side effects, Synthroid side effects, the Armour Thyroid and Synthroid dosing schedules, the Desiccated Thyroid (Armour Thyroid) and Levothyroxine guides, and Levothyroxine vs liothyronine: what is the difference between T4 and T3?.