Thyroid

What it is

The thyroid is a butterfly-shaped endocrine gland sitting at the front of the neck that produces two principal hormones — triiodothyronine (T3) and thyroxine (T4) — which regulate metabolism, energy use, heart rate, body temperature, and growth in virtually every organ system. Thyroid disorders are among the most common endocrine conditions in Canada, affecting an estimated 1 in 10 Canadians, with women diagnosed roughly 5–8 times more often than men. The gland is also sometimes called the thyroid gland or, in older clinical literature, the "master metabolic regulator."

Thyroid function is tightly controlled by the hypothalamic-pituitary-thyroid (HPT) axis. The hypothalamus releases thyrotropin-releasing hormone (TRH), which prompts the anterior pituitary to secrete thyroid-stimulating hormone (TSH). TSH then signals the thyroid to produce T4 (the storage form) and T3 (the biologically active form). Most circulating T3 is actually converted from T4 in peripheral tissues — primarily the liver and kidneys — by enzymes called deiodinases. This conversion step is clinically relevant because certain medications, illnesses, and nutritional deficiencies can impair it even when the thyroid gland itself is functioning normally.

In addition to T3 and T4, the thyroid contains C-cells that produce calcitonin, a hormone that opposes parathyroid hormone by inhibiting osteoclast activity and reducing blood calcium levels. Calcitonin's physiological importance in adults remains debated; patients who have had their thyroid removed show no consistent symptoms from absent calcitonin, though elevated levels can serve as a marker for medullary thyroid cancer.

Causes and mechanism

Thyroid hormone synthesis depends on adequate dietary iodine, which is absorbed in the small intestine and incorporated into T3 and T4. In Canada, iodized salt and dairy products are the primary iodine sources; frank iodine deficiency is uncommon but can occur in populations avoiding these foods. The most frequent cause of thyroid dysfunction in iodine-sufficient countries like Canada is autoimmune disease — Hashimoto's thyroiditis (causing hypothyroidism) and Graves' disease (causing hyperthyroidism) together account for the majority of cases. Other causes include thyroid nodules, radiation exposure, certain medications (amiodarone, lithium, interferon), and post-surgical or post-radioiodine ablation states.

Symptoms and diagnosis

Symptoms differ sharply depending on whether the thyroid is underactive or overactive:

Feature	Hypothyroidism (underactive)	Hyperthyroidism (overactive)
Weight	Gain	Loss
Heart rate	Slow (bradycardia)	Fast (tachycardia)
Temperature tolerance	Cold intolerance	Heat intolerance
Energy	Fatigue, sluggishness	Anxiety, restlessness
Bowel habits	Constipation	Diarrhea
Skin/hair	Dry skin, hair loss	Moist skin, fine hair
Menstrual cycle	Heavy or irregular periods	Light or absent periods

Diagnosis begins with a serum TSH test — the single most sensitive screening tool. A high TSH suggests hypothyroidism; a low TSH suggests hyperthyroidism. Free T4 and, where indicated, free T3 are measured to confirm severity. Thyroid antibody panels (anti-TPO, anti-thyroglobulin, TSH-receptor antibodies) help identify autoimmune causes. In Canada, TSH testing is available through LifeLabs and Dynacare at physician or nurse-practitioner request; most provincial health plans cover the test when ordered for clinical indications.

Treatment options

Treatment depends on the specific disorder:

• Hypothyroidism is treated with levothyroxine (synthetic T4), the most commonly prescribed thyroid medication in Canada. Dose is titrated to normalize TSH, typically targeting 0.5–2.5 mIU/L in most adults.
• Hyperthyroidism is managed with antithyroid drugs (methimazole, propylthiouracil), radioactive iodine ablation, or thyroidectomy, depending on cause and patient preference.
• Thyroid nodules are evaluated by ultrasound and, if suspicious, fine-needle aspiration biopsy; most are benign and require only monitoring.
• Thyroid cancer (papillary, follicular, medullary, anaplastic) is treated with surgery, radioiodine, and in some cases targeted therapies; five-year survival for papillary thyroid cancer — the most common type — exceeds 98% when caught early.

For patients seeking virtual care in Canada, several telemedicine platforms (Felix, Maple, Cleo, Phoenix, and others) can initiate thyroid screening and prescribe levothyroxine, though complex cases typically require in-person endocrinology referral.

When to see a clinician in Canada

See a family physician or nurse practitioner promptly if you notice an unexplained neck lump, rapid unintentional weight change, persistent fatigue unresponsive to rest, heart palpitations, or significant changes in menstrual patterns. Pregnant individuals warrant particular attention: untreated hypothyroidism during pregnancy is associated with impaired fetal neurodevelopment, and the Society of Obstetricians and Gynaecologists of Canada (SOGC) recommends TSH testing in pregnant patients with symptoms or risk factors. Referral to an endocrinologist is appropriate for confirmed hyperthyroidism, thyroid nodules larger than 1 cm, abnormal calcitonin levels, or any suspected thyroid malignancy.

Limitations and open questions

Research is still emerging on several fronts. The optimal TSH target range for older adults, pregnant individuals, and people with persistent symptoms despite "normal" lab values remains contested. The clinical benefit of adding T3 (liothyronine) to levothyroxine therapy for hypothyroid patients who feel unwell on T4 alone has been studied in multiple trials with inconsistent results; no Canadian guideline currently recommends routine combination therapy. Health Canada has not yet issued specific guidance on thyroid hormone testing intervals for subclinical hypothyroidism (TSH mildly elevated, free T4 normal), and practice varies across provinces. The role of calcitonin as a routine screening tool for medullary thyroid cancer is debated internationally, and Canadian centres do not universally measure it. Environmental factors — including certain pesticides and flame retardants classified as endocrine disruptors — are suspected to influence thyroid function, but dose-response data in humans remain limited.