Levothyroxine Dose: What You Need to Know About Dosage, Adjustments, and Safety

When your thyroid doesn’t make enough hormone, levothyroxine, a synthetic form of the thyroid hormone T4 used to treat hypothyroidism. Also known as synthroid, it’s one of the most prescribed medications in the world — but getting the dose right is where most people struggle. It’s not just about taking a pill. It’s about matching that pill to your body’s needs, which change over time.

Your levothyroxine dose, the amount of synthetic thyroid hormone prescribed to restore normal thyroid function depends on your age, weight, heart health, and whether you’re starting from zero or switching from another brand. A 25-year-old with no heart issues might start at 50 mcg, while a 70-year-old with atrial fibrillation might start at 25 mcg or less. Even small changes — like gaining 10 pounds, starting a new medication, or getting pregnant — can throw your dose off. Many people don’t realize their fatigue or weight gain isn’t just aging; it’s their thyroid dose needing a tweak.

Thyroid hormone replacement, the medical process of using levothyroxine to restore normal metabolic function in hypothyroid patients isn’t a set-it-and-forget-it deal. Blood tests (TSH and sometimes free T4) are your real guide. But even labs can lie — if you take your pill at night instead of morning, or eat breakfast right after, your numbers won’t reflect your true level. Timing matters. So does consistency. Skip a dose? Your body notices. Switch brands? Your doctor might need to recheck your levels.

And then there are the drugs that interfere. Iron, calcium, antacids, even soy milk can block absorption if taken too close to your pill. Some antidepressants and seizure meds can make your body clear levothyroxine faster. That’s why your dose might need adjusting every time you add or drop a medication. And if you’ve had thyroid cancer, your dose isn’t just to replace hormone — it’s to suppress TSH and keep cancer from coming back. That’s a whole different ballgame.

You’ll find posts here that break down how levothyroxine interacts with beta-blockers, how aging changes your needs, why some people still feel awful even with "normal" labs, and how to spot when your dose is too high or too low. No fluff. Just real-world advice from people who’ve been there — and doctors who’ve seen the patterns. Whether you’re new to levothyroxine or have been on it for years and still feel off, this collection gives you the tools to ask better questions and get better results.