Synthroid is simply a brand name for levothyroxine — the active ingredient, a synthetic version of the thyroid hormone T4, is identical. There are other brand names too (Levoxyl, Unithroid, Tirosint), and a generic version made by multiple manufacturers. The difference isn't what the drug is; it's how consistently it's made and priced.
If the active ingredient is the same, why does it matter which one you take?
Levothyroxine has what's called a narrow therapeutic index — the difference between too little and too much is small enough that minor variations in absorption can matter. Generic levothyroxine must fall within FDA bioequivalence standards, but different manufacturers' formulations (the inactive fillers and binders) can still absorb slightly differently from one person to the next. That's why some endocrinologists prefer a patient stay on the same manufacturer's version consistently, rather than switching between generic makers or brand-to-generic repeatedly.
Does switching from Synthroid to generic levothyroxine change your dose?
Usually not — they're prescribed at the same microgram dose. But because of that narrow-therapeutic-index sensitivity, it's standard practice to recheck TSH (thyroid-stimulating hormone) levels around 6 weeks after any brand or manufacturer switch, just to confirm your levels are still where they should be. It's a precaution, not a sign something is wrong.
Is one actually better than the other?
For most people, no — generic levothyroxine works the same as Synthroid for the same cost-conscious outcome, which is exactly why it's so widely prescribed. The brand name mainly matters for people who've had noticeable symptom changes after a past switch, or whose doctor has a specific reason (certain absorption issues, for example) to want formulation consistency.
What does this mean for cost?
This is where the difference is largest and most practical: generic levothyroxine is typically far cheaper than brand-name Synthroid in cash terms, often a fraction of the price for the identical active ingredient. Unless your doctor has a specific clinical reason to keep you on the brand name, generic is usually the straightforward choice.