Synthroid vs. Cytomel
See how Synthroid and Cytomel compare on approved uses, dosing, side effects, and cost.
Key Takeaways
Synthroid contains levothyroxine (T4), a slow, steady thyroid hormone taken once daily that can take four to six weeks to reach its full effect.
Cytomel contains liothyronine (T3), a faster-acting thyroid hormone that works within hours, so it's more often used short-term or for diagnostic testing than as routine daily therapy.
Both drugs carry FDA warnings against using thyroid hormone for weight loss, since doses beyond what the body needs can cause serious or life-threatening heart effects.
Synthroid vs. Cytomel Drug Summary
Synthroid
Synthroid is a synthetic form of thyroxine (T4), used to replace thyroid hormone in people with an underactive thyroid, pituitary, or hypothalamus. It's taken as a single oral tablet each morning, with the dose slowly adjusted based on blood tests every four to six weeks until thyroid levels settle into a normal range.
Cytomel
Cytomel is a synthetic form of triiodothyronine (T3), a faster-acting thyroid hormone used to treat the same types of hypothyroidism. It's taken as a once-daily oral tablet, but because it works and clears the body quickly, it's more often used short-term, for diagnostic thyroid testing, or as an add-on rather than as long-term first-line therapy.
Synthroid vs. Cytomel Side Effects
Synthroid
Cytomel
Synthroid vs. Cytomel Dosage
Synthroid
Cytomel
| STAGE | DOSE | WHEN |
| Starting dose | 1.6 mcg/kg/day | At the start |
| Maximum dose | Lowest effective dose | If needed |
| Dose changes | 3 increases | As tolerated |
| STAGE | DOSE | WHEN |
| Starting dose | 5 mcg once daily | At the start |
| Maximum dose | 75-100 mcg once daily | Days 1-7 |
| Dose changes | 3 increases | As tolerated |
Synthroid
| STAGE | DOSE | WHEN |
| Starting dose | 1.6 mcg/kg/day | At the start |
| Maximum dose | Lowest effective dose | If needed |
| Dose changes | 3 increases | As tolerated |
Cytomel
| STAGE | DOSE | WHEN |
| Starting dose | 5 mcg once daily | At the start |
| Maximum dose | 75-100 mcg once daily | Days 1-7 |
| Dose changes | 3 increases | As tolerated |
Synthroid vs. Cytomel Indications
Synthroid
Synthroid is approved to replace thyroid hormone in primary, secondary, and tertiary hypothyroidism, including congenital cases in infants and children. It's also used to suppress TSH after treatment for certain types of thyroid cancer. It is not approved for weight loss or for shrinking non-cancerous thyroid nodules or goiters.
Primary, secondary, or tertiary hypothyroidism
Congenital or acquired hypothyroidism in children
TSH suppression after thyroid cancer treatment
Not approved for weight loss
Cytomel
Cytomel is approved to replace thyroid hormone in primary, secondary, and tertiary hypothyroidism. It's also used to suppress TSH in well-differentiated thyroid cancer and as a diagnostic aid in thyroid suppression testing. Like Synthroid, it is not approved for weight loss or for shrinking non-cancerous thyroid nodules.
Primary, secondary, or tertiary hypothyroidism
Diagnostic aid for thyroid suppression testing
TSH suppression in well-differentiated thyroid cancer
Not approved for weight loss
Synthroid vs. Cytomel Pros and Cons
Synthroid
Synthroid's long half-life keeps hormone levels steady on one daily dose, and its 12 tablet strengths allow fine-tuned dosing. As the most prescribed thyroid hormone brand, it's well covered by insurance, though pharmacies also stock many generic alternatives.
Pros
Long half-life gives stable, once-daily dosing
12 precise tablet strengths for fine-tuning
Most-prescribed thyroid hormone, decades of use
Cons
Takes 4-6 weeks to reach full effect
Requires repeat blood tests during titration
Narrow therapeutic index; avoid switching brands
Absorption affected by food, coffee, and supplements
Cytomel
Cytomel's active T3 hormone acts within hours, which makes it useful for thyroid scan preparation and suppression testing rather than everyday maintenance. It carries a boxed warning against use for weight loss, since doses beyond what the body needs can cause serious cardiac harm.
Pros
Acts within hours, unlike slower-acting T4 drugs
Useful for thyroid scan and withdrawal protocols
Doubles as a diagnostic suppression-test aid
Cons
Boxed warning: not for weight loss
Higher risk of arrhythmia, especially in older adults
Shorter duration means levels fluctuate more
Less commonly used as routine daily therapy
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