Hypothyroidism is the underactivity of the thyroid gland,
this is when the thyroid produces less hormones then expected known as
hypo-secretion, which causes a slow metabolism due to hypo-responsiveness of
target cells. Hypothyroidism is an endocrine disorder which can be divided into
primary which is caused by diseases in the target glands or secondary which is caused
by diseases in the pituitary gland. There are two main causes of hypothyroidism
the first being autoimmune and the second is post-therapeutic. There are three
different types of hypothyroidism such as primary (Giotrous) hypothyroidism,
the common causes for this is Hashimoto (an autoimmune disease), endemic iodine
deficiency, postpartum thyroiditis, subacute thyroiditis and drug induce such
as treatment for hyperthyroidism which may lead to hypothyroidism. The common
causes for primary (non-goitrous) hypothyroidism include; surgical ablation, radioactive
iodine therapy, atrophic myxedema, external radiation and sporadic critinism.

The common cause for secondary hypothyroidism include; pituitary disease due to
low secretion of TSH and hypothalamic disease, this happens when the
hypothalamus produces an inadequate amount of thyrotropin-releasing hormone
(TRH). There are other further common causes of hypothyroidism include diet,
congenital dysfunction, chemical exposure, lack of T4 being converted to T3.

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