Thyroid hormone replacement·#1 most prescribed in Canada

Synthroidlevothyroxine sodium

Also known as: Euthyrox, Eltroxin

Synthroid (levothyroxine) treats hypothyroidism by replacing thyroid hormone. Learn about dosing, side effects, and coverage at Pill4Me Pharmacy Chilliwack.

Prescriptions (Canada 2024)

20,500,000 prescriptions

Drug class

Thyroid hormone replacement

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 it 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 & 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 to take it

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.

Side effects

Common side effects

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

Serious side effects — seek care immediately

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 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.

BC PharmaCare coverage

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 details change — confirm with your Pill4Me pharmacist or visit BC PharmaCare.

Frequently asked questions

Why do I have to take Synthroid on an empty stomach?

Food — especially high-fibre foods like cereal, bran, and coffee — can reduce how much levothyroxine your body absorbs by 30–40%. Taking it 30–60 minutes before breakfast on an empty stomach ensures your body gets the full dose. If this is difficult, talk to your pharmacist — some patients take it at bedtime with consistent results.

Can I switch between brand-name Synthroid and a generic?

Generally it is best to stay on the same product (brand or generic) consistently. Small differences in potency between manufacturers can cause your TSH to shift outside your target range. If you do switch, ask your doctor to recheck your TSH 6–8 weeks later.

What should I do if I miss a dose of Synthroid?

Take it as soon as you remember, as long as it is still on an empty stomach and before your first meal. If you have already eaten, skip the missed dose and take your next dose the following morning as normal. Do not double up — taking two doses at once will not help and may cause side effects.

Why does my doctor keep changing my dose?

Levothyroxine has a very narrow therapeutic range. Your dose is based on your TSH blood test, and the right dose can change with age, weight change, pregnancy, menopause, other new medications, or changes in how you take it. Most patients are stable once the right dose is found, typically within 6–12 months of starting.

Will Synthroid help me lose weight?

If your hypothyroidism was causing unexplained weight gain, treating it with levothyroxine may help you return to your normal weight. However, levothyroxine does not cause weight loss in people who have normal thyroid function, and it should never be used for weight loss purposes.

Related medications

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Questions about Synthroid?

Our Chilliwack pharmacists can review your medications, check for interactions, and answer any questions — at no charge.

Call 604-705-3644 Transfer my prescription →

Medical disclaimer: This page is for general educational purposes only and does not constitute medical advice. Always consult your pharmacist or prescriber before making any changes to your medications. BC PharmaCare coverage criteria are subject to change.