Overactive thyroid

An overactive thyroid (hyperthyroidism) occurs when the thyroid gland produces too much thyroid hormone. This accelerates the body's metabolism and can affect multiple organ systems.

The most common cause is Graves' disease, an autoimmune condition. Other causes include thyroid nodules and thyroiditis (inflammation of the thyroid).

Symptoms may include:

Unexplained weight loss despite a normal or increased appetite
A rapid or irregular heartbeat
Anxiety, irritability and difficulty sleeping
Tremor, sweating and heat intolerance

Triiodothyronine (T3) is one of the key thyroid hormones and is often elevated in hyperthyroidism. A blood test for TSH, free T4 and T3 together provides a comprehensive picture of thyroid function.

Causes of Overactive thyroid

The most common cause of hyperthyroidism is Graves' disease, an autoimmune condition in which the immune system produces antibodies (TSH receptor antibodies) that stimulate the thyroid gland to overproduce thyroid hormones.

Other causes include:

Toxic multinodular goitre, in which several nodules in the thyroid gland become overactive, more common in older adults
Toxic adenoma, a single overactive thyroid nodule
Thyroiditis (inflammation of the thyroid), which can cause a temporary release of stored thyroid hormone. This may follow a viral infection (subacute thyroiditis) or occur after pregnancy (postpartum thyroiditis)
Excessive iodine intake, including from certain medications such as amiodarone
Over-treatment with levothyroxine for hypothyroidism

Graves' disease accounts for around 75% of hyperthyroidism cases. It is more common in women and may be triggered by stress, infection or hormonal changes in genetically predisposed individuals.

Tests that can help check this condition

Triiodothyronine (FT3)

Learn more on the official NHS page .

Medical review

Dr. Shahzaib Ahmad MBBS, BSc, MRCP, FRCA, FFICM

Clinical Supervisor - Vitall|GMC 7495062|Profile

Dr Shahzaib Ahmad is a Consultant in Intensive Care Medicine with subspecialty experience in cardiac critical care and neurocritical care. He has extensive experience managing complex acute and cardiovascular conditions in hospital intensive care settings. Dr Ahmad supports Vitall in reviewing health content for clinical accuracy and alignment with current medical practice.

Reviewed on 22/05/2026

Next review due 22/05/2027

Review focus: Blood biomarkers, laboratory testing methodology, and biochemical interpretation.

This content has been reviewed for medical and scientific accuracy but does not replace advice from a qualified healthcare professional.