Hyperthyroidism

What it is

Hyperthyroidism — also called overactive thyroid — affects approximately 1 to 2% of the population and occurs when the thyroid gland produces more T3 (triiodothyronine) and T4 (thyroxine) than the body requires, driving every metabolic process into an accelerated state. Women are diagnosed significantly more often than men, and the condition is most commonly identified in women between ages 20 and 40, though it can appear at any age. The most frequent underlying cause is Graves' disease, an autoimmune condition responsible for 60 to 80% of cases. Left untreated, hyperthyroidism raises the risk of atrial fibrillation, osteoporosis, heart failure, and — in severe uncontrolled cases — a life-threatening emergency called thyroid storm. In Canada, initial workup is typically ordered through primary care and processed at national laboratory networks such as LifeLabs or Dynacare; endocrinology referral follows when the diagnosis is confirmed or the cause is unclear.

Causes and mechanism

Thyroid hormones are normally regulated by thyroid-stimulating hormone (TSH) released from the pituitary gland. Hyperthyroidism arises when that feedback loop is bypassed or overwhelmed. The main causes differ in mechanism and management:

Cause	Mechanism	Typical patient
Graves' disease (60–80% of cases)	Autoantibodies stimulate the TSH receptor, driving continuous unregulated hormone production	Women 20–40; autoimmune history
Toxic multinodular goitre	Multiple nodules produce hormone autonomously, independent of TSH	Adults over 60
Toxic adenoma (Plummer's disease)	Single autonomously functioning nodule overproduces hormone	Any adult
Subacute thyroiditis	Post-viral thyroid inflammation releases stored hormone transiently	Any age; often follows respiratory illness
Postpartum thyroiditis	Autoimmune inflammation within 12 months of delivery; transient hyperthyroid phase	Postpartum women
Iodine excess / amiodarone	High iodine load triggers unregulated hormone synthesis	Cardiac patients on amiodarone
Levothyroxine over-treatment	Exogenous hormone excess from dose miscalibration	Hypothyroid patients on replacement
TSH-secreting pituitary adenoma	Pituitary tumour drives excess TSH output	Rare; any age

Graves' disease is distinguishable from other causes by the presence of TSH receptor antibodies (TRAb) and, in roughly 25% of patients, thyroid eye disease (Graves' ophthalmopathy) — a feature not seen in nodular or thyroiditis-related hyperthyroidism.

Symptoms and diagnosis

Because thyroid hormones regulate nearly every organ system, symptoms are wide-ranging. The most common include rapid or irregular heartbeat, unintentional weight loss despite a normal or increased appetite, heat intolerance, excessive sweating, fine hand tremor, anxiety, irritability, insomnia, fatigue, muscle weakness, and frequent bowel movements. Women may notice irregular or absent periods; men may experience reduced libido or erectile dysfunction. A visible or palpable goitre is present in many but not all cases.

Diagnosis follows a stepwise approach:

1. TSH — the most sensitive screening test; a suppressed (very low) TSH is the hallmark finding.
2. Free T4 and free T3 — elevated in overt hyperthyroidism; free T3 alone may be elevated in T3 toxicosis.
3. TSH receptor antibodies (TRAb) — positive in Graves' disease; helps avoid unnecessary imaging.
4. Thyroid uptake scan — differentiates Graves' disease (diffuse uptake) from toxic nodules (focal uptake) and thyroiditis (low or absent uptake).
5. Thyroid ultrasound — assesses gland size, vascularity, and nodule characteristics.

Treatment options

Treatment is tailored to the underlying cause, severity, patient age, pregnancy status, and preference.

Symptom control: Beta-blockers (propranolol, atenolol) provide rapid relief of palpitations, tremor, and anxiety while definitive treatment takes effect. They do not lower hormone levels.

Anti-thyroid medications: Methimazole blocks thyroid hormone synthesis and is the standard first-line medical treatment, taken for 12 to 18 months with the goal of inducing remission. Propylthiouracil (PTU) is preferred in the first trimester of pregnancy due to methimazole's teratogenic risk. Both are available in Canada under provincial drug benefit formularies, though coverage criteria vary by province.

Radioactive iodine (RAI) therapy: An oral dose of radioactive iodine selectively destroys overactive thyroid tissue. It is highly effective but results in permanent hypothyroidism in most patients, requiring lifelong levothyroxine replacement. RAI is contraindicated in pregnancy and is generally avoided when significant thyroid eye disease is present, as it may worsen ophthalmopathy.

Surgery (thyroidectomy): Removal of all or most of the thyroid is preferred for large goitres, coexisting suspicious nodules, significant eye disease, or when medications are not tolerated. Lifelong levothyroxine follows. In Canada, thyroidectomy is performed in hospital under provincial health insurance; wait times vary by region.

Thyroiditis-related hyperthyroidism: Because hormone release in thyroiditis comes from stored hormone rather than ongoing synthesis, anti-thyroid drugs are ineffective. Management is supportive — beta-blockers for symptom relief — and the condition typically resolves within weeks to months.

When to see a clinician in Canada

See your family physician or nurse practitioner if you notice unexplained weight loss despite a good appetite, a rapid or irregular heartbeat, persistent hand tremor, new or worsening anxiety, excessive sweating, heat intolerance, or a visible swelling at the front of the neck. A simple TSH blood test ordered through primary care is the appropriate first step and is covered under all provincial health plans.

Go to an emergency department immediately if you develop a very rapid heart rate combined with fever, agitation, confusion, or vomiting. These signs may indicate thyroid storm — a rare but life-threatening complication of severe uncontrolled hyperthyroidism that requires urgent hospital treatment.

Limitations and open questions

Research is still emerging on the optimal duration of anti-thyroid drug therapy and the factors that best predict remission in Graves' disease; current guidelines recommend 12 to 18 months, but some evidence supports longer courses. The long-term cardiovascular risk of subclinical hyperthyroidism (suppressed TSH with normal free T4) remains debated, and Health Canada has not issued specific guidance on screening thresholds for subclinical disease in asymptomatic adults. The relationship between radioactive iodine treatment and worsening of thyroid eye disease is recognized but incompletely understood, and management of moderate-to-severe ophthalmopathy is an active area of clinical investigation. Optimal management of hyperthyroidism in pregnancy — balancing fetal safety with maternal disease control — continues to evolve, and decisions should be made in consultation with a maternal-fetal medicine specialist or endocrinologist experienced in thyroid disease.