What are the causes of hypothyroidism?
Primary hypothyroidism is due to intrinsic thyroid gland failure. The most common causes are irradiation, surgery, and “burnt out” thyroiditis. Other causes include iodine deficiency or excess, amiodarone, lithium, and anti-thyroid medications. The laboratory abnormalities in pri-mary hypothyroidism are elevated levels of TSH and low T3 and T4 levels.
Secondary hypothyroidism is due to inadequate levels of TSH with a normal thyroid gland. This can be caused by pituitary failure or pituitary surgery. The laboratory abnormalities in secondary hypothyroidism are low levels of T3, T4, and TSH but elevated levels of TRH.
Tertiary hypothyroidism is due to hypothalamic failure. The laboratory abnormalities in tertiary hypothyroidism are low levels of T3, T4, TSH, and TRH.