Thyroid Treatment Side Effects
All effective treatments can cause side effects. Knowing what to expect — and which symptoms need urgent attention — helps you use thyroid treatment safely.
This page summarises the side effects of the main thyroid treatments.
Last medically reviewed: 15 November 2025
Levothyroxine
Taken at the correct dose, levothyroxine rarely causes side effects. Problems usually come from taking too much, which can cause palpitations, anxiety, insomnia, sweating and weight loss, or too little, which leaves fatigue and cold intolerance.
Dose-related effects are managed by adjusting the dose with blood-test monitoring — never by changing it yourself.
Antithyroid medicines
Mild side effects include rash, joint aches and taste changes. Rare but serious effects include a dangerous fall in white blood cells (agranulocytosis) and liver inflammation.
Seek urgent medical attention if you develop a fever, sore throat or unusual tiredness while taking these medicines — these can be warning signs.
Radioactive iodine
Radioactive iodine is generally well tolerated. Some people notice a tender neck or dry mouth for a short time. The main long-term effect is an underactive thyroid, which is expected and treated with levothyroxine.
Thyroid surgery
After surgery, temporary hoarseness or a change in voice can occur from nerve effects, and low calcium can cause tingling or cramps. These usually settle but are monitored closely.
Permanent complications are uncommon with experienced surgeons, and lifelong levothyroxine is needed after total thyroidectomy.
Frequently asked questions
Can levothyroxine cause side effects?
At the correct dose it rarely does. Symptoms such as palpitations or fatigue usually mean the dose is too high or too low and needs adjusting with blood tests.
Which symptoms need urgent attention?
While taking antithyroid medicines, a fever, sore throat or marked tiredness should be assessed urgently, as they can signal a serious drop in white blood cells.
Are side effects permanent?
Most side effects are temporary or dose-related and can be corrected. Permanent complications are uncommon and are reduced by careful monitoring and experienced care.
References & further reading
This page is based on established medical knowledge and guidelines from authoritative sources. Always consult a qualified clinician for personal medical advice.
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