Synthroidlevothyroxine sodium
Also sold as: Euthyrox, Eltroxin
Daily thyroid hormone replacement for an underactive thyroid — timing and consistency matter more than with most medications.
Quick answer
- What it is
- A synthetic form of thyroxine (T4), the hormone an underactive thyroid gland no longer makes enough of.
- What it's used for
- Hypothyroidism, prevention of goitre, and part of the treatment plan for some thyroid cancers.
- Worth knowing
- Take it on an empty stomach 30–60 minutes before breakfast, and keep calcium, iron and antacids at least 4 hours apart from it.
- Main safety point
- Too much causes palpitations, insomnia and tremor. Dose is set by blood TSH levels, so it is adjusted for you individually — never change it on your own.
Already taking Synthroid? Pill4Me pharmacists in Chilliwack can help.
Drug class
Thyroid hormone replacement
Use in Canada
20,500,000 prescriptions
DIN (Canada)
02230734, 02230735 +3 more
What is Synthroid?
Synthroid is used to treat hypothyroidism — an underactive thyroid gland that does not produce enough thyroid hormone on its own. It is also used to prevent goitre (enlarged thyroid) and as part of treatment for certain types of thyroid cancer.
How Synthroid works
Levothyroxine is a synthetic version of thyroxine (T4), the main hormone your thyroid gland normally produces. After you take it, your body converts it into the active form (T3) that your cells need to control metabolism, energy levels, heart rate, and body temperature.
Forms and strengths
Tablets: 25 mcg, 50 mcg, 75 mcg, 88 mcg, 100 mcg, 112 mcg, 125 mcg, 137 mcg, 150 mcg, 175 mcg, 200 mcg, 300 mcg. Generic levothyroxine is available in the same strengths.
How Synthroid is taken
Take on an empty stomach, 30–60 minutes before your first meal of the day. Swallow with a full glass of water. Separate from calcium supplements, iron supplements, and antacids by at least 4 hours, as these significantly reduce absorption. Dose is highly individual and adjusted based on blood TSH levels.
This is general information, not a dose for you. Dosing depends on the reason you are taking it, your kidney and liver function, your age, and everything else you take — follow the directions on your own label, and ask us if they differ from what you read here.
If you miss a dose
If you remember later the same day, you can usually take it then. If it is almost time for the next day's dose, skip the missed one and carry on — do not take two doses to catch up. Levothyroxine has a long half-life, so a single missed dose is unlikely to cause harm, but repeated missed doses will shift your TSH out of range. Tell your pharmacist if you are missing doses often.
Side effects
Common — usually manageable
- When dose is correct, most people have no side effects
- Signs of too high a dose: palpitations, rapid or irregular heartbeat
- Insomnia or feeling wired
- Excessive sweating or heat sensitivity
- Unintended weight loss
- Shakiness or tremor in the hands
- Anxiety or irritability
- Diarrhea or more frequent bowel movements
Get medical help for these
- Chest pain or angina — call your doctor immediately
- Very rapid, pounding, or irregular heartbeat
- Shortness of breath or difficulty breathing
- Long-term overtreatment can cause bone loss (osteoporosis) — your doctor will monitor
- Severe allergic reaction: hives, swelling, difficulty breathing — go to emergency
Important warnings
Take on an empty stomach — food, especially high-fibre foods, significantly reduces how much your body absorbs
Calcium and iron supplements must be separated by 4 hours — they bind to levothyroxine in your gut
Do not switch between brand (Synthroid) and generic without telling your doctor — small potency differences can affect your TSH
Narrow therapeutic index — even small dose changes can push levels too high or too low; regular TSH testing is essential
Pregnancy changes thyroid hormone requirements — notify your doctor immediately if you become pregnant
Who should take extra care
- Older adults
- Doses are usually started lower and increased more slowly, because a dose that is too high can strain the heart.
- Heart disease
- Angina or a recent heart attack changes how levothyroxine is started — this needs physician supervision.
- Pregnancy
- Thyroid requirements normally rise in pregnancy and TSH needs closer monitoring. Do not stop the medication — talk to your prescriber early.
- Adrenal insufficiency
- Untreated adrenal problems must be treated first.
- Supplements
- Calcium, iron, and multivitamins containing them block absorption. So do antacids and some cholesterol binders.
Who should not take it
- Untreated adrenal insufficiency (adrenal gland problem) — treat adrenal problem first
- Recent heart attack or acute myocarditis
- Allergy to levothyroxine or any ingredient in the tablet
- Thyrotoxicosis (already overactive thyroid) — unless used for specific cancer treatment
Drug interactions
Calcium carbonate and iron supplements reduce absorption by up to 40% — separate by at least 4 hours. Cholestyramine and colestipol (cholesterol medications) significantly reduce absorption. Warfarin sensitivity increases — INR monitoring needed. Certain antidepressants (sertraline, carbamazepine) and diabetes medications may require dose adjustments. Antacids containing aluminum, magnesium, or calcium also reduce absorption.
This is not a complete list. If you take several medications, or buy anything over the counter, a pharmacist medication review is the reliable way to catch interactions that matter.
Coverage in British Columbia
Synthroid (levothyroxine) may be covered under BC PharmaCare depending on your plan. Covered under Fair PharmaCare for eligible BC residents. Long-term care residents are covered under Plan B (Long-term Care). Income assistance recipients are covered 100% under Income Assistance (Plan C). First Nations members may have 100% coverage under First Nations Health Authority (Plan W). Coverage amounts vary — Fair PharmaCare covers 70–100% after your annual deductible depending on your income. Call us at 604-705-3644 and we'll check your specific coverage in minutes. BC PharmaCare covers the generic version at 100% of the approved price. If you choose the brand name instead of the generic, you pay the difference in cost out of pocket. Your pharmacist can dispense the generic automatically unless your doctor has specified 'no substitution' on the prescription. Important: levothyroxine has a narrow therapeutic index — small differences between brand and generic can affect thyroid levels. If you switch between brand and generic, your doctor may want to recheck your TSH. Talk to your Pill4Me pharmacist before switching. Coverage information is a general guide only. Your actual coverage depends on your PharmaCare plan and eligibility. We check coverage for free — call 604-705-3644 or ask at the counter.
Coverage may depend on indication, PharmaCare plan and Special Authority requirements. Ask Pill4Me Pharmacy to check your coverage, or see BC PharmaCare.
Frequently asked questions
Why do I have to take Synthroid on an empty stomach?
Can I switch between brand-name Synthroid and a generic?
What should I do if I miss a dose of Synthroid?
Why does my doctor keep changing my dose?
Will Synthroid help me lose weight?
Related reading
Conditions it’s used for
Already taking Synthroid?
Move your prescription to Pill4Me and we handle the rest — we contact your current pharmacy, and refills, reminders and free local delivery come with it.
Pill4Me Pharmacy is at 5625 Promontory Rd Unit 101, Chilliwack, BC. We deliver free to Chilliwack, Sardis, Promontory, Vedder and Yarrow, and you can reach a pharmacist directly at 604-705-3644.