Disclaimer: This article is for informational purposes only and does not constitute medical advice. It is not intended to be a substitute for professional medical advice, diagnosis, or treatment. Always seek the advice of a physician or other qualified health provider with any questions you may have regarding a medical condition.
Overview
Synthroid is a brand name for levothyroxine sodium, a man-made form of the thyroid hormone thyroxine (T4). It is made by BGP Pharma ULC and comes as tablets in 12 strengths, from 25 mcg to 300 mcg. Health Canada has authorized it to treat hypothyroidism (an underactive thyroid) in people of any age, including during pregnancy, and for some thyroid cancers.
Thyroid hormone replacement is usually taken for life. The right dose is different for everyone and is set with regular blood tests. Other levothyroxine products are sold in Canada, but they should not be swapped for Synthroid without repeat blood tests and a dose check.
Synthroid needs a prescription. BC residents can speak with a BC-licensed doctor by phone through Avee Health to ask whether it is appropriate for them. The doctor decides based on your health history and thyroid blood tests, and may order tests or ask for an in-person assessment first. A prescription is not guaranteed.
In Canada, Synthroid is authorized for:
- Hypothyroidism of any cause, at any age including newborns, and during pregnancy. It is not used for the temporary hypothyroidism that can occur while recovering from subacute thyroiditis.
- Some thyroid cancers (well-differentiated papillary or follicular cancer), together with surgery and radioactive iodine
It is not for weight loss or obesity, and it is not a treatment for infertility unless the infertility is caused by hypothyroidism.
Levothyroxine sodium
Thyroid hormone
hormones
Prescription required
Available
How Synthroid Works
The thyroid gland makes hormones that control how the body uses energy and keep many organs working normally. In hypothyroidism the gland does not make enough. Levothyroxine is a man-made copy of thyroxine (T4), the main hormone the thyroid makes. The body converts it to the active hormone T3 as needed.
T4 leaves the body slowly, with a half-life of about 6 to 7 days, so blood levels change gradually. That is why the dose is checked with a TSH (thyroid stimulating hormone) blood test several weeks after each change.
Dosage Instructions of Synthroid
Your doctor sets your dose. It depends on your age, weight, heart health, the cause of your hypothyroidism, how long and how severe your symptoms have been, and your blood tests. The usual starting points in the Canadian product monograph are:
- Younger, healthy adults: about 1.7 mcg per kg of body weight once a day, usually started at the full dose
- Most adults over 50, and adults under 50 with heart disease: 25 to 50 mcg a day, increased gradually every 6 to 8 weeks as needed
- Adults over 50 with heart disease: 12.5 to 25 mcg a day, increased gradually every 4 to 6 weeks. Synthroid does not come in a 12.5 mcg tablet, so another product may be used for that dose.
- Children and babies: the dose is based on age and weight and changes as the child grows
Blood tests: TSH is usually checked 6 to 8 weeks after starting or changing the dose, and the dose is adjusted in small steps. Once it is stable, it is checked at least once a year.
How to take it: once a day at the same time, on an empty stomach, 30 minutes to 1 hour before breakfast. Swallow the tablet whole with a full glass of water. Take it at least 4 hours apart from calcium, iron, antacids and other products that block its absorption. Food and drink change how much you absorb, so take it the same way every day.
For babies and children who cannot swallow tablets, the tablet can be crushed and mixed into 5 to 10 mL of water, breast milk or non-soy formula and given right away by spoon or dropper, or sprinkled on a little apple sauce. Do not store the mixture, and avoid foods or formula high in soy, fibre or iron.
Missed dose: take it as soon as you remember. If it is almost time for your next dose, skip the missed one. Do not take two doses at once. If you miss 2 or more doses in a row, check with your doctor.
Storage Instructions
Store Synthroid at room temperature, 15°C to 25°C, in the original package, protected from light and moisture. Keep it out of the reach and sight of children, and do not use it after the expiry date.
Side Effects of Synthroid
Side effects of levothyroxine are mostly signs that the dose is too high. No percentages are available. Possible effects include:
- Nervousness, anxiety, restlessness or quick changes in mood
- Trouble sleeping, headache or tiredness
- Sweating, flushing, fever or feeling too warm
- Diarrhea, nausea, vomiting or stomach cramps
- Tremor or muscle weakness
- Changes in appetite, weight or menstrual periods, and temporary hair loss
A dose that is too low can leave symptoms of hypothyroidism. Tell your doctor about any of these effects, because the dose may need adjusting.
Get medical help right away if you have:
- Chest pain, a fast or irregular heartbeat, palpitations or shortness of breath
- Signs of a heart attack, such as crushing chest pain spreading to the left arm or jaw, with sweating or nausea
- Signs of a serious allergic reaction: rash, hives, swelling of the face, lips, tongue or throat, or trouble swallowing or breathing
- A seizure
- In children: bad headaches with vision problems or double vision, or a limp with hip, groin or knee pain
Important Safety Information About Synthroid
- Not for weight loss: thyroid hormones, alone or with other medicines, must not be used to treat obesity or for weight loss. Larger doses can cause serious or life-threatening effects, especially with stimulant-type appetite suppressants.
- Do not take Synthroid if you are allergic to levothyroxine or any ingredient, have an untreated overactive thyroid, have recently had a heart attack or heart inflammation (myocarditis or pancarditis), have untreated adrenal insufficiency, or are pregnant and taking medicine for an overactive thyroid.
- Heart disease and older adults: treatment starts low and goes up slowly. Too much levothyroxine can cause chest pain and heart rhythm problems, such as atrial fibrillation.
- Pregnancy: do not stop Synthroid. Tell your doctor as soon as you know you are pregnant, because the dose often needs to go up and TSH is checked more often.
- Breastfeeding: only small amounts pass into breast milk. Keep taking it, because normal thyroid levels help maintain milk production.
- Bones: long-term treatment has been linked to lower bone density in women, especially after menopause on higher doses. The lowest effective dose is used.
- Diabetes: starting or changing thyroid hormone can change how much diabetes medicine or insulin you need. Watch your blood sugar closely.
- Mental health: if you are at risk of psychotic disorders, treatment starts at a low dose and goes up slowly.
- Surgery: tell your surgeon or dentist that you take Synthroid.
- Ingredients: Synthroid tablets contain lactose and acacia and are gluten-free. The 50 mcg tablet has no colour dyes.
Drug Interactions
Many medicines and supplements interact with levothyroxine. Tell your doctor and pharmacist about everything you take. The main ones are:
- Products that block absorption (take at least 4 hours apart): calcium carbonate, iron (ferrous sulfate), antacids with aluminum or magnesium, sucralfate, cholestyramine, colestipol, sevelamer, lanthanum and sodium polystyrene sulfonate. Orlistat and proton pump inhibitors can also lower levothyroxine levels.
- Foods: soy flour and soy infant formula, cottonseed meal, walnuts, calcium-fortified orange juice and dietary fibre can reduce absorption.
- Medicines that can raise the dose you need: carbamazepine, phenytoin, phenobarbital, rifampin, ritonavir, sertraline, imatinib, sunitinib and St. John's wort
- Warfarin and similar blood thinners: their effect can increase, so clotting tests need close monitoring
- Diabetes medicines and insulin: doses may need to change when levothyroxine is started, changed or stopped
- Digoxin: its effect may be reduced
- Tricyclic antidepressants, decongestants and other stimulant-type medicines, and ketamine: higher risk of heart rhythm problems, a fast heartbeat or high blood pressure
- Amiodarone, lithium and iodine, including iodine contrast used for scans: these can change how the thyroid works
Biotin supplements, including some hair and nail products, can give falsely high or low thyroid test results. Your doctor may ask you to stop biotin at least 2 days before the test.
Brands & DIN Number of Synthroid
Brand Names
Synthroid
Key Takeaways
- 1Synthroid is levothyroxine, a man-made thyroid hormone taken once a day to treat hypothyroidism.
- 2The dose is set and adjusted with TSH blood tests, usually 6 to 8 weeks after each change and then at least once a year.
- 3Take it on an empty stomach, 30 to 60 minutes before breakfast, and at least 4 hours apart from calcium, iron and antacids.
- 4Do not switch between levothyroxine products without your doctor knowing, because a repeat blood test may be needed.
- 5Thyroid hormones must not be used for weight loss. Large doses can cause serious or life-threatening effects.
- 6Chest pain, a fast or irregular heartbeat or shortness of breath need medical help right away.
- 7Keep taking it during pregnancy and breastfeeding. The dose often goes up in pregnancy.
Frequently Asked Questions
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Avee Editorial
Avee Editorial Team