What is Hypothyroidism?

Hypothyroidism is primarily characterized by a failure of the thyroid gland to produce normal amounts of thyroid hormones. Iodine deficiency is the most common factor in worldwide hypothyroidism incidence. Exposure to Iodine-131 (I-131, aka radioiodine, a radioisotope of iodine used for medical and pharmaceutical purposes) is also a known risk factor for this disease. Depending on the “source of the problem” (hypothalamus, pituitary or thyroid gland), hypothyroidism is divided into three types:

  • Primary Hypothyroidism – the most common form of hypothyroidism, this condition is characterized by the failure of the thyroid gland to produce sufficient thyroid hormone (mainly as thyroxine (T4) and smaller amounts of tri-iodothyronine (T3)). The most common form of primary hypothyroidism is Hashimoto’s Disease which is an autoimmune disorder.
  • Secondary Hypothyroidism – typically caused by the failure of the pituitary gland to create enough thyroid-stimulating hormone (TSH) to in turn stimulate the thyroid into producing adequate amounts of T3 and T4.
  • Tertiary Hypothyroidism – usually caused by a deficiency in the hypothalamus causing a failure to regulate the pituitary gland through the secretion of thyroid-releasing hormone (TRH).
  • Subclinical Hypothyroidism – more common among women with an estimated prevalence range of 3–8% and increasing with age. There is a certain amount of controversy regarding the management of this subtype of the disease as this condition is viewed somewhat differently depending on the medical perspective:
    Traditional [medicine] perspective which most often uses the term “subclinical” defines this type of hypothyroidism as one characterized by elevated TSH levels coupled with normal T4 and T3 levels with minimal or no obvious signs and symptoms of low thyroid function.
    Alternative [medicine] perspective has a divergent definition, whereby patients might present with signs and symptoms of low thyroid function without any lab values being “outside the normal range”.
    The most important implication of subclinical hypothyroidism is the high likelihood of progression to primary hypothyroidism. The possibility that it is a cardiovascular risk factor has been a subject of debate.
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