Causes of Primary Hypothyroidism

A certain percentage of hypothyroidism is present at birth (approximately 1 in 4000 newborns has a severe thyroid function deficiency), however the onset of the disease can present at various stages of life and the causes can vary somewhat with the age of onset. While a detailed analysis of the specific cause and effect relationships in thyroid disease would be a far too complex topic to present on a single web site, the following is a characterization of the general cause categories for hypothyroidism:

Congenital Hypothyroidism

It is important to stress the fact that while “congenital” implies a condition present at birth; it does not necessarily translate to “hereditary”. In many cases; congenital hypothyroidism is acquired during a fetal developmental stage.

  1. Thyroid Agenesis: the failure of the thyroid gland to develop fully by birth. This can be caused by environmental factors such as any exposure to radioactive iodine during stages critical to thyroid development. The incidence of thyroid agenesis maybe as high as 30% of all patients with congenital hypothyroidism.
  2. Genetic: very often presents as a structurally normal thyroid gland with defects affecting thyroxine or triiodothyronine synthesis. At other times, a genetic mutation causes a “peripheral resistance” to thyroid hormone whereby target tissues show no response to normal thyroid hormone production.
  3. Iodine Deficiency: dietary iodine deficiency continues to be an important public health problem in many regions around the globe. This deficiency presents a real danger to proper thyroid development during the prenatal stages as well as the post-birth nursing period.

Transient Hypothyroidisms due to inflammation of the thyroid gland (Thyroiditis)

  1. Subacute Thyroiditis is a result of a viral infection that stimulates the immune system to release massive amounts of lymphocytes (white blood cells, part of the immune function) directed against the thyroid tissue. This results in pain, tenderness, and signs and symptoms of exposure to too much thyroid hormone or thyrotoxicosis (increased nervousness, restlessness , heart palpations, sweating) followed by a hypothyroid phase which may last a few weeks to several months. Eventually, patients will revert to a euthyroid (normal) state, but some will remain in a permanent hypothyroid state.
  2. Postpartum Thyroiditis occurs within six weeks to six months after giving birth in 6% to 8.8% of postpartum women. It initially manifests with the typical signs and symptoms of hyperthyroidism (increased nervousness, restlessness, heart palpations, sweating, etc) followed by a hypothyroid phase which lasts for about one year postpartum until the patient returns to a euthyroid (normal) state. Some women (23%) develop subsequent permanent hypothyroid.

Autoimmune Hypothyroidism

Antibody mediated immune processes can sometimes lead to gradual destruction of the thyroid gland by the various immune system components (most common in U.S.):

  1. Hashimoto’s Disease (also known as Hashimoto’s Thyroiditis as well as Chronic Lymphocytic Thyroiditis): with an average of 1-1.5 per 1000 people affected by this disease, it is the most common cause of primary hypothyroidism in North America -is found to be the mechanism for hypothyroidism in 90% of the cases. Hashimoto’s Disease occurs with a significantly higher frequency among women than in men (between 10:1 and 20:1) and tends to be most prevalent between 45 and 65 years of age. This autoimmune disorder initially results in a lack of optimal thyroid function due to inflammation and scaring damage to the thyroid tissue. At some point, enough thyroid tissue will have been destroyed so that what remains is no longer capable of producing adequate amounts of the thyroid hormones necessary to meet the body’s needs.


(Inadvertent adverse effects or complications resulting from medical treatment)

  1. Exposure to the radioisotope Iodine 131 as a result of radioactive iodine ablation therapy following a diagnosis of thyroid cancer or hyperthyroidism. Approximately 80% to 90% of patients will develop hypothyroidism within two to three months of this treatment.
  2. Surgical removal of the thyroid gland: Total Thyroidectomy (total removal of the thyroid gland) renders the patient hypothyroid in the immediate post surgery periods. 60% to 80% of patient’s undergone subtotal or partial removal of the thyroid gland will develop hypothyroidism within a year after surgery.
  3. Overdose of anti-thyroid medication such as Methmimazole.

Other Causes:

  1. Medications: Some commonly prescribed medications can affect thyroid function. Medications such as anti-inflammatory agents, anti-depressants, diabetic medications, hypertensive medications, pain medications, antacid and cholesterol medications. Many of these medications may disrupt thyroid hormone physiology in a way that may not cause changes in the serum TSH.
  2. Exposure to radiation: inadvertent exposure of the head and neck area to radiation may lead to thyroid tissue destruction.
  3. Iodine deficiency. Iodine is used in the synthesis of thyroid hormones. Dietary iodine deficiency results in decreased production of thyroid hormones. Iodine deficiency is now rare in the U.S. as a result of widespread distribution of iodine enriched foods. The incidence of iodine deficiency occurs almost exclusively in developing nations.
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