Short answer: Cytomel is liothyronine, T3 alone, an FDA-approved tablet dosed in micrograms.1 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.2 Both labels list hypothyroidism and carry a boxed warning against use for weight loss. Neither label reports a trial against the other.
| Cytomel | Armour Thyroid | |
|---|---|---|
| What is in it | Liothyronine sodium (T3)1 | Dried pig thyroid: 38 mcg T4 and 9 mcg T3 per grain (60 mg)2 |
| FDA status | FDA-approved prescribing information1 | Label states it has not been approved by FDA as a new drug2 |
| Strengths | 5, 25 and 50 mcg tablets1 | 15 mg (1/4 grain) to 300 mg (5 grains)2 |
| Listed uses | Hypothyroidism; with surgery and radioiodine in thyroid cancer; thyroid suppression test1 | Hypothyroidism; TSH suppression in certain goiters, nodules, thyroiditis and thyroid cancer2 |
| Adult starting dose | 25 mcg once daily, up 25 mcg every 1 or 2 weeks; usually 25 to 75 mcg a day1 | Usually 30 mg, up 15 mg every 2 to 3 weeks; most need 60 to 120 mg a day2 |
| Older people or heart disease | Start 5 mcg, up in 5 mcg steps1 | Start low at 15 to 30 mg2 |
| Side effects | Mainly signs of too much thyroid hormone: fast heartbeat, nervousness, tremor, heat intolerance, weight loss1 | Rare apart from signs of too much thyroid hormone: chest pain, fast pulse, palpitations, sweating, nervousness2 |
| Boxed warning | Not for obesity or weight loss1 | Not for obesity or weight loss2 |
What is in the tablet. Cytomel is T3 only. Armour Thyroid contains both T4 and T3 from pig thyroid glands, and its label notes T3 is about four times as potent as T4 microgram for microgram.12 The Cytomel label says that when switching from levothyroxine, levothyroxine is stopped and Cytomel starts at a low dose, because Cytomel acts quickly and the previous medicine's effects may last several weeks.1 Doses are on different scales (micrograms vs grains or mg) and are not swapped one for one without your prescriber.
Both warn about heart effects in older people or those with heart disease, effects on diabetes control, and the need for lower starting doses. The Cytomel label lists uncorrected adrenal insufficiency as its one contraindication, says tablets are not used for myxedema coma, and warns that over-replacement can lower bone density.1 The Armour Thyroid label also lists untreated overactive thyroid, notes that biotin can throw off thyroid blood tests, and says that because it comes from animal tissue, contamination with animal viruses cannot be ruled out, with no cases of disease transmission reported.2
Which, if either, fits is for your prescriber. More detail: Cytomel side effects, Armour Thyroid side effects, the Cytomel and Armour Thyroid dosing schedules, Armour Thyroid vs Synthroid: how does dried thyroid differ from levothyroxine? and Levothyroxine vs liothyronine: what is the difference between T4 and T3?.