Hypothyroidism (an underactive thyroid gland) is by far the most common thyroid problem. In hypothyroidism the thyroid gland does not make enough thyroid hormones (T4 and T3) and the body’s metabolism slows down. (1)
We diagnose hypothyroidism by ordering two blood tests (serum TSH and serum Free T4). Usually the TSH is elevated above normal and the Free T4 is below the normal range.
The number one cause of hyporthyroidism is Hashimoto’s thyroiditis. In this disease the thyroid gland and its ability to make thyroid homones are damaged by attack from the body’s own immune system.
We diagnose Hashimoto’s disease by ordering the microsomal antibody test (also called a number of ther names including the thyroid peroxidase antibody). The microsomal antibody test will be above the normal range in Hashimoto’s disease.
Other causes of hypothyroidism include amiodarone (a drug used for heart rhythm problems). Around 20% of patients taking amiodarone, will developan elevated serum TSH.
Lithium, which is used to treat bipolar disorder and severe depression, causes hypothyroidism in about 20 to 30% of patients taking it.
Many other medicines can cause hypothyroidism and so it is best to research each drug that a patient with hypothyroidism is taking.
Too much or too little iodine in the diet can cause hypothyroidism.
Treatment of an overactive thyroid gland (hyperthyroidism) by surgery, radioactive iodine, or medicines can cause hypothyroidism.
Disease of the pituitary gland (the gland which makes TSH) can lead to hypothyroidism. This diagnosis would suggested by a low serum TSH test along with a low Free T4 test.
But again, the most common cause of hypothyroidism (an underactive thyroid gland) is Hashimoto’s disease.
(1) The Cleveland Clinic Guide to Thyroid Disorders. 2009. Mario Skugor MD.