- An underactive thyroid can cause modest weight gain, often partly fluid; treating it usually helps somewhat.
- TSH is the first-line blood test in Canada.
- Levothyroxine is the standard treatment and is very effective when dosed correctly.
- Using thyroid hormone (T3) to lose weight without a diagnosis is dangerous for the heart and bones.
Thyroid problems are common, especially in women, and are often blamed for weight changes. This guide explains how the thyroid affects weight, how testing and treatment work in Canada, and what to watch for.
How the thyroid affects weight
- Hypothyroidism (underactive): slows metabolism; can cause fatigue, cold intolerance, constipation, dry skin and modest weight gain
- Hyperthyroidism (overactive): speeds metabolism; can cause weight loss, rapid heart rate, anxiety, heat intolerance and tremor
Weight gain from hypothyroidism is usually a few kilograms, often partly fluid, rather than the large gains people sometimes expect.
Testing in Canada
TSH (thyroid-stimulating hormone) is the standard first test. If it’s abnormal, free T4 and sometimes antibodies are checked. Your family doctor can order these, and they’re generally covered when medically indicated.
Treatment
Hypothyroidism is treated with levothyroxine, a synthetic T4, sold in Canada under names including Synthroid and Eltroxin. Dosing is adjusted using TSH tests every six to eight weeks until stable. Hyperthyroidism is treated with medication, radioactive iodine or surgery.
Take levothyroxine consistently, usually on an empty stomach, and separate it from calcium, iron and some other supplements, which reduce absorption.
Why T3 for weight loss is dangerous
Liothyronine (T3, sold as Cytomel) is sometimes used without a diagnosis to speed up metabolism for fat loss. In people with normal thyroid function, it can cause rapid heart rate, irregular heart rhythms, anxiety, muscle loss and bone loss, and it suppresses your own thyroid while you take it. It’s a prescription drug in Canada. See our weight loss myths guide.
Losing weight with a thyroid condition
- Get treated and stable first
- Focus on protein, fibre and a moderate calorie deficit
- Strength train to protect muscle and metabolism
- Be patient: progress may be slightly slower, but it happens
The Thyroid Foundation of Canada offers patient information and support.
Frequently asked questions
Can hypothyroidism make you gain weight?
Yes, usually a few kilograms, often partly fluid.
What thyroid test should I ask for in Canada?
TSH is the first-line test; free T4 is added if TSH is abnormal.
Will treating my thyroid help me lose weight?
Often somewhat, but diet and activity still matter most.
Is T3 safe for weight loss?
No. Without a diagnosis it can harm the heart and bones and suppress your own thyroid.
This article is general information for adults in Canada, not medical advice. Talk to a doctor or pharmacist before using any prescription drug, anabolic steroid or research compound, and get bloodwork done. If you’re in crisis, call or text 9-8-8 (Suicide Crisis Helpline) any time.
