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
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.