Can hypothyroidism cause weight gain?

Yes, though the weight gain is usually modest — typically 2–5 kg — and reflects fluid retention and a reduced metabolic rate rather than significant fat accumulation. Severe obesity caused by hypothyroidism alone is uncommon. Correcting thyroid hormone levels with levothyroxine often leads to partial reversal of this weight gain, but it rarely resolves obesity when other contributing factors are present.

Does everyone with hypothyroidism need medication?

Not necessarily. Subclinical hypothyroidism — elevated TSH with normal free T4 and no symptoms — does not always require treatment. Most guidelines recommend starting levothyroxine when TSH is consistently above 10 mIU/L, when significant symptoms are present, or during pregnancy. For TSH in the 5–10 mIU/L range without symptoms, annual TSH monitoring is a reasonable approach, though some clinicians treat based on symptoms even at lower TSH levels.

Can hypothyroidism cause depression?

Yes. Thyroid hormones influence serotonin metabolism and overall brain function, and low levels are associated with low mood, depression, cognitive slowing, and poor motivation. In some people, depression is the most prominent presenting symptom. Treating the underlying thyroid condition often improves mood significantly, though additional antidepressant therapy is sometimes needed as well. Canadian clinical guidelines recommend checking TSH as part of the workup for any new-onset depression.

Is it safe to take levothyroxine long-term?

Yes. Levothyroxine replaces a hormone the body can no longer produce in adequate amounts, and at the correct dose it has no significant adverse effects — it is one of the safest long-term medications in use. The main risk is over-treatment: excess levothyroxine can cause symptoms of hyperthyroidism and, over time, reduce bone density and increase the risk of atrial fibrillation. This is why TSH should be checked every 6–12 months once a stable dose is established.

Is hypothyroidism testing and treatment covered in Canada?

TSH blood tests are covered under provincial health insurance in all Canadian provinces and territories when ordered by a licensed clinician. Levothyroxine is listed on most provincial formularies — including the Ontario Drug Benefit and BC PharmaCare — and is available at low or no cost to eligible patients. Coverage for liothyronine (T3) and desiccated thyroid extract varies by province and individual benefit plan, so patients should confirm with their pharmacist or provincial drug plan.

Hypothyroidism — Glossary

What it is

Hypothyroidism is an endocrine condition in which the thyroid gland produces insufficient thyroid hormone, slowing the body's metabolism and affecting virtually every organ system. Also called underactive thyroid, it affects approximately 2–3% of the general population and up to 5% of adults over 60, with women diagnosed 8–10 times more often than men. In Canada, thyroid function testing is widely available through provincial laboratory networks such as LifeLabs and Dynacare, and most provincial drug benefit plans cover levothyroxine — the standard treatment — at low or no cost.

Hypothyroidism exists on a spectrum. Subclinical hypothyroidism is defined as an elevated TSH with a normal free T4 and few or no symptoms; it is more prevalent than overt disease. Overt hypothyroidism involves both an elevated TSH and a low free T4, with clear clinical symptoms. Both forms warrant monitoring, and overt hypothyroidism requires treatment in nearly all cases.

Form	TSH	Free T4	Symptoms	Typical management
Subclinical	Elevated (typically 4.5–10 mIU/L)	Normal	Minimal or absent	Monitor annually; treat if TSH >10 or symptoms present
Overt	Elevated (often >10 mIU/L)	Low	Present	Levothyroxine replacement
Central (secondary)	Low or normal	Low	Present	Treat underlying pituitary/hypothalamic cause

Causes and mechanism

The thyroid gland synthesizes thyroxine (T4) and triiodothyronine (T3) under direction from thyroid-stimulating hormone (TSH), which is secreted by the pituitary gland. When thyroid output falls, the pituitary responds by raising TSH — the basis of the primary screening test.

Hashimoto's thyroiditis (also called chronic lymphocytic thyroiditis or autoimmune thyroiditis) is the leading cause in iodine-sufficient countries, including Canada. It is an autoimmune condition in which antibodies and white blood cells progressively destroy thyroid tissue, gradually reducing hormone output over years or decades.

Other causes include:

Iodine deficiency — the most common cause globally, though rare in Canada due to iodized salt
Thyroid surgery — partial or total thyroidectomy for cancer, goitre, or hyperthyroidism
Radioactive iodine therapy — used to treat hyperthyroidism; frequently results in permanent hypothyroidism
Radiation to the head or neck — damages thyroid tissue as a treatment side effect
Medications — amiodarone, lithium, interferon-alpha, and tyrosine kinase inhibitors all impair thyroid hormone production or release
Postpartum thyroiditis — autoimmune thyroid inflammation following childbirth; can cause a transient hypothyroid phase in the months after delivery
Central hypothyroidism — rare; caused by pituitary or hypothalamic dysfunction that fails to produce adequate TSH or TRH
Congenital hypothyroidism — present from birth; detected through mandatory newborn screening programs in all Canadian provinces

Symptoms and diagnosis

Symptoms develop gradually and are often attributed to stress or aging, which delays diagnosis. They reflect a system-wide metabolic slowdown:

Persistent fatigue and lethargy
Weight gain or difficulty losing weight (typically 2–5 kg, largely from fluid retention)
Cold intolerance
Dry skin, brittle nails, and hair thinning or loss
Constipation
Slow heart rate (bradycardia)
Low mood, depression, or cognitive slowing ("brain fog")
Muscle aches and weakness
Facial, hand, or foot puffiness
Heavy or irregular menstrual periods
Elevated cholesterol
Hoarse voice

In severe, untreated cases, myxoedema — a life-threatening state of profound hypothyroidism — can develop, though this is rare with access to routine testing.

Diagnostic workup:

TSH — the primary and most sensitive screening test; an elevated result signals the pituitary is compensating for low thyroid output
Free T4 — a low result alongside elevated TSH confirms overt hypothyroidism
Thyroid peroxidase antibodies (TPO-Ab) — when positive, confirms an autoimmune (Hashimoto's) cause
Lipid panel — elevated LDL cholesterol is common in untreated hypothyroidism
Full blood count — mild anaemia is sometimes present

Treatment options

Hypothyroidism is among the most effectively managed hormonal conditions. Treatment replaces the hormone the thyroid can no longer produce in adequate amounts.

Levothyroxine (synthetic T4) is the standard first-line treatment. Taken once daily on an empty stomach — ideally 30–60 minutes before breakfast — it provides synthetic T4 that the body converts to active T3 as needed. Food, calcium supplements, iron, and certain medications reduce absorption and should be separated by at least four hours. Dosing is titrated to TSH, with most treated patients targeting a TSH of 0.5–2.5 mIU/L, though the optimal target varies by age and clinical context.

Combination T4/T3 therapy — using liothyronine (synthetic T3) alongside levothyroxine, or desiccated thyroid extract (which contains both hormones naturally) — may be considered for patients who remain symptomatic despite a normalized TSH on levothyroxine alone. Evidence supporting combination therapy is mixed, and it is not a first-line recommendation in most guidelines, but it is an option some endocrinologists discuss with patients on an individual basis.

Subclinical hypothyroidism: Treatment is generally recommended when TSH consistently exceeds 10 mIU/L, or at lower elevations in pregnant women or those with significant symptoms. For TSH in the 5–10 mIU/L range without symptoms, annual monitoring is a reasonable alternative to immediate treatment.

Pregnancy: Thyroid hormone requirements increase by 25–50% during pregnancy. Women with known hypothyroidism who become pregnant should increase their levothyroxine dose promptly and have TSH monitored every 4–6 weeks through the first trimester, as untreated or undertreated hypothyroidism is associated with impaired fetal brain development.

When to see a clinician in Canada

See a family physician or nurse practitioner if you have persistent unexplained fatigue, unexplained weight gain, cold intolerance, hair thinning, low mood or cognitive difficulties, heavy or irregular periods, or constipation and facial puffiness without a clear dietary cause. A TSH blood test ordered through any provincial lab is the appropriate first step.

People with a first-degree family history of Hashimoto's thyroiditis or other autoimmune thyroid disease have a meaningfully higher risk and should mention this to their clinician. Women who are pregnant or planning to conceive should have thyroid function assessed early, given the consequences of untreated hypothyroidism for fetal development.

For Canadians seeking virtual assessment, several telehealth platforms — including Maple, Felix, Cleo, and others — can order TSH testing and initiate levothyroxine prescriptions where clinically appropriate, though complex cases (suspected central hypothyroidism, combination therapy, or thyroid cancer history) warrant referral to an endocrinologist.

Limitations and open questions

Research is still emerging on several aspects of hypothyroidism management. The optimal TSH target range for treated patients — particularly older adults, those with cardiovascular disease, and those who are pregnant — remains an area of active debate, and guidelines from the American Thyroid Association (2014) and the Endocrine Society (2012) differ in their recommendations.

The role of combination T4/T3 therapy is not settled. Randomized controlled trials have produced inconsistent results, and Health Canada has not issued specific guidance on liothyronine use in hypothyroidism. Desiccated thyroid extract is available in Canada but is not a first-line recommendation in any major guideline.

Whether treating subclinical hypothyroidism in older adults (TSH 5–10 mIU/L, no symptoms) improves outcomes — including cardiovascular risk or quality of life — is unresolved. A 2019 Cochrane review found no clear benefit of treatment in this group, and clinical practice varies across Canadian provinces. Clinicians and patients should make this decision collaboratively, weighing individual risk factors.

Hypothyroidism