Problems with the thyroid can be caused by:

• iodine deficiency
• autoimmune diseases, in which your immune system attacks your own body, leading either to hyperthyroidism (caused by Graves’ disease) or hypothyroidism (caused by Hashimoto's thyroiditis)
• inflammation of the thyroid (which may or may not cause pain), caused by a virus or bacteria
• nodules, or non-cancerous lumps, inside the thyroid
• cancerous tumours on the thyroid gland
• certain medical treatments, including radiation therapy, thyroid surgery, and some medications
• some genetic disorders.

Occasionally, pregnancy can cause thyroid problems to start or get worse. If left untreated, this can cause problems for mother and baby, including miscarriage, premature birth or stillbirth.