Etiology of Hypothyroidism

• Excluding thyroidectomy and radioactive iodine (131I) ablation:

  • The most common causes of hypothyroidism in the adult are:

    • Hashimoto’s thyroiditis (Chronic Lymphocytic Thyroiditis)

    • The hypothyroid phase of subacute thyroiditis

  • Because the long-term treatment is very different:

    • The clinicians must distinguish between these conditions.

• The common causes of low circulating thyroid hormone levels are:

  • Primary hypothyroidism  (thyroid failure with elevated TSH):Hashimoto’s thyroiditis (chronic lymphocytic thyroiditis)
    • Hypothyroid phase of painful subacute thyroiditis:Pseudo-granulomatous–De Quervain’s thyroiditis
    • Hypothyroid phase of painless lymphocytic thyroiditis
    • Hypothyroid phase of postpartum thyroiditis
    • Radioactive iodine ablation
    • Thyroidectomy
    • Head and neck radiation
    • Drugs:Lithium
      • Amiodarone
      • Interleukin
      • Interferon
      • Propylthiouracil / methimazole
      • Iodine excess in patients with thyroiditis
    • Iodine deficiency (uncommon in the United States)
    • Biosynthetic defects (rare and presents in childhood)
    • Congenital hypothyroidism (rare and presents in childhood)
  • Secondary (hypothyroidism with low or inappropriately normal TSH):Pituitary dysfunction:Pituitary damage from tumor, surgery, and / or radiation
  • Tertiary:Hypothalamic damage from:Tumor and / or radiation

Management

Euthyroid patients with positive thyroid antibody titers can typically be

monitored without the institution of thyroid hormone replacement

therapy.26 However, there are some data that pregnant patients with

positive thyroid antibody titers may have improved pregnancy out-

comes and reduced complications with the institution of LT4 replace-

ment therapy. In non-pregnant patients with hypothyroidism, there are

standard recommendations for treatment and monitoring.27 This usu-

ally consist of LT4 therapy and TSH and FT4 monitoring every 6 weeks

with adjustments in LT4 dosing until the TSH is within the goal range

(typically 1 to 3 uIU/mL) although a higher target range is considered

acceptable in the elder

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