The i-pill (levonorgestrel) contains a high dose of hormones which can cause temporary fluid retention and bloating, mimicking rapid weight gain. This is temporary and typically resolves within a few days. While you are taking Thyronorm (levothyroxine) for your thyroid, a single dose of the emergency contraceptive will not cause permanent weight gain or disrupt your thyroid function long-term. If you continue to experience weight changes or feel unwell, it is a good idea to have your thyroid levels checked by your doctor to ensure your dosage is still correct.