Side effects of radioactive iodine for hyperthyroidism

Radioactive iodine is taken up by thyroid cells and gradually reduces their activity. During the early phase, symptoms such as palpitations, sweating, heat intolerance, anxiety, loose stools or difficulty sleeping may persist or temporarily feel stronger.

The intended reduction in thyroid activity can eventually go further than needed, leaving too little thyroid hormone. This is called an underactive thyroid and may cause tiredness, feeling cold, constipation, dry skin or low mood. It is managed with clinician-prescribed hormone replacement when appropriate.

Attend every blood-test appointment, even if you feel well, because symptoms alone cannot show how thyroid levels are changing. Take or stop other thyroid medicines only according to the nuclear-medicine or endocrinology team's instructions.

Follow the service's radiation-safety advice about close contact, hygiene and pregnancy. Tell the team promptly if you are pregnant, breastfeeding, or planning pregnancy. Keep a record of new palpitations, [Excessive Sweating](/symptoms/excessive-sweating/) or [Feeling Anxious](/symptoms/feeling-anxious/).

Seek urgent help for chest pain, fainting, severe breathlessness, a very rapid or irregular heartbeat, or marked agitation, confusion or weakness. These symptoms need prompt assessment rather than waiting for the next planned test.

Contact your treating team if diarrhoea, [Night Sweats](/symptoms/night-sweats/), [Palpitations](/symptoms/palpitations/) or other symptoms are persistent or worsening. Your team may adjust monitoring or treatment for [Hyperthyroidism](/conditions/hyperthyroidism/).

Ask when your blood tests will be checked, which symptoms should prompt a call, and how long to follow radiation-safety precautions. Ask what would happen if your thyroid becomes underactive and how pregnancy plans affect treatment.