The thyroid is a gland in your neck, and thyroid cancer is cancer that affects this gland.
I don’t know any side effects, but I can tell you that one of the treatments for thyroid cancer is pretty clever: the thyroid accumulates iodine, so doctors give patients radioactive iodine. This means that the radioactivity collects in the thyroid and kills the cancer cells, but doesn’t do too much damage to the rest of the body. Afterwards, the radioactive iodine can be ‘flushed out’ of the thyroid by taking normal iodine. I think it’s really cool!
Thyroid cancer is when you have a tumour in your thyroid which is malignant. The side effects are a hard nodule on the thyroid, or an enlarged lymph node. Later symptoms can be pain or even changes in voice! Over, or under, activity of the thyroid is also associated with larger tumours. They can be treated with surgery, radiotherapy or chemotherapy.