Types of hypothyroidism
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Types of Hypothyroidism: An Overview
Primary Hypothyroidism: Most Common Form
Primary hypothyroidism is the most prevalent type, accounting for over 99% of all cases. It occurs when the thyroid gland itself fails to produce sufficient thyroid hormones. Common causes include chronic lymphocytic thyroiditis (Hashimoto's disease), radioiodine thyroid ablation, thyroidectomy, high-dose head and neck radiation therapy, and certain medications like lithium and amiodarone Almandoz2012Chelimsky2005. This form is characterized by elevated thyroid-stimulating hormone (TSH) levels and low levels of thyroxine (T4) .
Central Hypothyroidism: Pituitary or Hypothalamic Origin
Central hypothyroidism results from insufficient production of TSH due to disorders of the pituitary gland or hypothalamus. Causes include tumors, inflammatory conditions, infiltrative diseases, infections, pituitary surgery, and head trauma . Unlike primary hypothyroidism, central hypothyroidism is marked by low T4 levels but normal or low TSH levels .
Subclinical Hypothyroidism: Mild Thyroid Failure
Subclinical hypothyroidism is characterized by elevated TSH levels with normal free T4 levels. It is often asymptomatic but can progress to overt hypothyroidism. This condition is more prevalent in elderly populations and those with type 2 diabetes mellitus (T2DM) or metabolic syndrome (MetS) Almandoz2012Brenta2019Han2015. Subclinical hypothyroidism is associated with increased risks of cardiovascular diseases and diabetic complications Brenta2019Han2015.
Consumptive Hypothyroidism: Rare and Severe
Consumptive hypothyroidism is a rare form caused by excessive inactivation of thyroid hormones due to high levels of type 3 iodothyronine deiodinase (D3) in tumors. This condition has been observed in both children and adults with vascular tumors, fibrous tumors, and gastrointestinal stromal tumors Ruppe2005Pasa2017. It requires high doses of thyroid hormone replacement and often tumor-directed therapy Ruppe2005Pasa2017.
Hypothyroidism in Childhood: Cretinism and Juvenile Myxedema
In children, hypothyroidism can manifest as cretinism, which can be sporadic or endemic. Sporadic cretinism is often due to developmental defects of the thyroid gland, while endemic cretinism is linked to iodine deficiency in goitrous families Hutchison1954Braid1951. Juvenile myxedema is another term used to describe hypothyroidism in older children, often presenting with similar symptoms to adult hypothyroidism but with additional growth and developmental delays .
Hashimoto Thyroiditis: Autoimmune Hypothyroidism
Hashimoto thyroiditis (HT) is the most common autoimmune cause of hypothyroidism. It includes several variants such as the fibrous variant, IgG4-related variant, juvenile form, Hashitoxicosis, and painless thyroiditis. HT is characterized by lymphocytic infiltration of the thyroid gland, leading to atrophy and hypothyroidism . Diagnosis involves detecting antibodies against thyroid antigens and reduced thyroid echogenicity on ultrasound .
Conclusion
Hypothyroidism encompasses a range of conditions from primary and central hypothyroidism to subclinical and consumptive forms. Each type has distinct etiologies, clinical features, and management strategies. Understanding these differences is crucial for accurate diagnosis and effective treatment.
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Hypothyroidism as an inborn error of metabolism.
Hypothyroidism is an inborn error of metabolism, with three classic types: sporadic, athyroidic, and goitrogenic, with a third type occurring due to therapeutic use or accidental ingestion of substances with a goitrogenic action.
CASE FINDING FOR HYPOTHYROIDISM SHOULD INCLUDE TYPE 2 DIABETES AND METABOLIC SYNDROME PATIENTS: A LATIN AMERICAN THYROID SOCIETY (LATS) POSITION STATEMENT.
The Latin American Thyroid Society recommends aggressive case finding of hypothyroidism in both type 2 diabetes mellitus and metabolic syndrome patients, as thyroid function is affected in these conditions.
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