The most common cause for this is autoimmune thyroiditis where the body makes antibodies against the thyroid. After removing a thyroid lobe, up to 20% of patients may become hypothyroid and require thyroid hormone replacement.
After removing the whole thyroid gland, all patients require thyroid hormone replacement.
The treatment is thyroid hormone (Thyroxine) in tablet form. This is very well tolerated as it is the same as the hormone produced by the thyroid gland.