There was a sudden increases in the size of Thyroid swelling along with pain. Most likely cause is

. A painful nodule, sudden appearance, or rapid enlargement of a nodule raises suspicion of carcinoma but is usually due to haemorrhage into a simple nodule. Comment : more common among the given options is haemorrhage. other points to consider: Pain is not a common feature of thyroid swellings. Thyroiditis can present with a painful gland. Anaplastic carcinoma can cause local pain and pain referred to the ear if it infiltrates surrounding structures Source : Bailey and love 26 th edition Pg : 748