Those at Risk for Underactive Thyroid 

The Mayo Clinic mentions a number of risk factors for hypothyroidism. Women are more likely to have an underactive thyroid than men, especially older women. Women who have been pregnant or delivered in the past six months may be at a higher risk. Those with autoimmune diseases or close relatives who had autoimmune diseases may be at higher risk. Treatment with radioactive iodine, anti-thyroid medications or having the thyroid removed may put patients at greater risk. Radiation exposure to the upper chest or neck also puts people at greater risk. 

Causes of Underactive Thyroid 

There are many different causes of underactive thyroid. Iodine deficiency, which is a lack of iodine in one's diet, may cause hypothyroidism. Another cause is Hashimoto's disease, an autoimmune disorder that causes immune cells to attack and destroy cells at the thyroid gland. 

Pituitary gland dysfunction could cause hypothyroidism when the master gland does not make enough thyroid-stimulating hormone. This is a rare cause of underactive thyroid, however, and is often associated with a tumor on the pituitary gland. 

Simply getting the thyroid removed to treat hyperthyroidism (an overactive thyroid) can cause hypothyroidism. Radioactive iodine and anti-thyroid medications used to treat hyperthyroidism also could lead to hypothyroidism.