Hypothyroidism, Adult

BASICS

DESCRIPTION

  • Metabolic state resulting from decreased levels of free thyroid hormone or from resistance to hormone action.
  • Subclinical: elevated TSH, normal free T4 (1)
  • Overt: elevated TSH, subnormal free T4

EPIDEMIOLOGY

Incidence

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

Prevalence

  • Overt hypothyroidism: 11.7% of the U.S. population (2)
  • Subclinical hypothyroidism 4–20% of adults (1)

ETIOLOGY AND PATHOPHYSIOLOGY

  • Primary: abnormality at the thyroid gland (>95% of cases)
    • Most common cause worldwide: environmental iodine deficiency
    • Most common cause in the United States: Hashimoto thyroiditis (chronic autoimmune thyroiditis) (2)
    • Postablative/posttherapeutic
    • Transient hypothyroidism: de Quervain syndrome (viral), postpartum (3)
    • Drug use: propylthiouracil, methimazole, lithium, amiodarone, antiepileptic drugs, and newer chemotherapeutic agents such as tyrosine kinase inhibitors (sunitinib), interleukin-2, or interferon-α
  • Secondary is deficiency of TSH from pituitary gland, tertiary deficiency of TRH from hypothalamus
  • Peripheral: T3 and T4 excessively degraded by ectopically produced type 3 iodothyronine deiodinase (rare)

Genetics

  • TSH levels and T4 levels have a genetic factor with a chance of 40–60% being transferred from parent to child (2).
  • Hashimoto’s thyroiditis patients have a 4.9% change of having a first-degree relative with the same diagnosis (4).

RISK FACTORS

  • Personal or family history of autoimmune diseases
  • External head or neck irradiation
  • Treatment with amiodarone, lithium, interferon-α, sunitinib, or sorafenib
  • Down syndrome or Turner syndrome

GENERAL PREVENTION

  • Iodized salt has significantly decreased iodine-deficient causes of hypothyroidism in western countries.
  • Those needing additional supplementations include people consuming unbalanced vegetable-based or salt-restricted diets as well as young females and those pregnant or lactating.

COMMONLY ASSOCIATED CONDITIONS

Autoimmune disorders if cause

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