Hypothyroidism, Adult

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  • Clinical and metabolic state resulting from decreased levels of free thyroid hormone or from resistance to hormone action
  • Primary (intrinsic thyroid disease) or central (secondary or tertiary resulting from hypothalamic-pituitary disease)
  • Subclinical: serum TSH above the upper reference limit with a normal free thyroxine (T4) and normal hypothalamic-pituitary-thyroid axis (1)
  • Overt: elevated TSH, usually >10 mIU/L with a subnormal free T4


  • Women: 3.5/1,000 persons per year
  • Men: 0.6/1,000 persons per year

  • The National Health and Nutrition Examination Survey III (NHANES III), subclinical hypothyroidism 9.3%, overt 0.3–3.7% in an unselected U.S. population age >12 years, with upper limit TSH 4.5
  • Framingham study, 5.9% of women and 2.3% of men age >60 years had a serum TSH >10 mIU/L.

Etiology and Pathophysiology

  • Primary: abnormality at the thyroid gland (>95% of cases)
  • Most common cause worldwide: environmental iodine deficiency (1)[A]
  • Most common cause in the United States: Hashimoto thyroiditis (chronic autoimmune thyroiditis)
    • Hashimoto is characterized pathologically by infiltration of the thyroid with sensitized T lymphocytes and serologically by circulating antithyroid antibodies.
    • Autoimmunity to the thyroid gland is an inherited defect in immune surveillance, leading to abnormal regulation of immune responsiveness or alteration of presenting antigen in the thyroid (1).
  • Postablative/posttherapeutic: follows radioactive iodine therapy or hemithyroidectomy for hyperthyroidism; radiotherapy or surgery for thyroid cancer, benign nodular thyroid disease, or neck malignancies
  • Transient hypothyroidism: de Quervain syndrome (viral), postpartum, silent thyroiditis (2)
  • Drug use: propylthiouracil, methimazole, lithium, amiodarone, antiepileptic drugs, and newer chemotherapeutic agents such as tyrosine kinase inhibitors (sunitinib), interleukin-2, or interferon-α
  • Central: hypothyroidism due to insufficient stimulation by TSH of an otherwise normal thyroid gland
  • Can be secondary (level of the pituitary) or tertiary (level of the hypothalamus)
  • Etiology involves genetic defects, tumors, vascular, empty sella syndrome, inflammatory, infiltrative, iatrogenic, posttrauma, or drug related.

Risk Factors

  • Women >60 years of age
  • Personal or family history of autoimmune diseases
  • Pregnant women or those with previous postpartum thyroiditis
  • Previous head or neck irradiation
  • Past history of thyroid dysfunction or thyroid surgery
  • Abnormal thyroid examination, presence of goiter and/or TPOAb positivity
  • Treatment with amiodarone, lithium, interferon-α, sunitinib, or sorafenib
  • Down syndrome or Turner syndrome

Commonly Associated Conditions

  • Diabetes mellitus type 1 and 2
  • Pernicious anemia
  • Primary adrenal failure (Addison disease)
  • Myasthenia gravis
  • Celiac disease
  • Rheumatoid arthritis
  • Systemic lupus erythematosus
  • Vitiligo
  • Depression
  • Genetic syndromes that have multiple autoimmune endocrinopathies (MAE) such as type 1 MAE and type 2 MAE

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