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Source: Jonklaas J, Talbert RL. Thyroid Disorders. In: DiPiro JT, Talbert RL, Yee GC, Matzke GR, Wells BG, Posey LM, eds. Pharmacotherapy: A Pathophysiologic Approach. 8th ed. http://accesspharmacy.com/content.aspx?aid=7991868. Accessed June 22, 2012.

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  • Clinical and biochemical syndrome resulting from decreased thyroid hormone production.

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  • Hashimoto's disease
  • Iatrogenic hypothyroidism
  • Iodine deficiency
  • Enzyme defects
  • Thyroid hypoplasia
  • Goitrogens
  • Pituitary disease
  • Hypothalamic disease

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  • Most patients have primary hypothyroidism due to thyroid gland failure from chronic autoimmune thyroiditis (Hashimoto’s disease).
  • Defects in suppressor T-lymphocyte function lead to survival of mutating clone of helper T lymphocytes that produce thyroid antibodies.
  • Iatrogenic hypothyroidism follows exposure to excessive amounts of radiation (radioiodine or external radiation) or after total thyroidectomy.
  • Other causes of primary hypothyroidism include:
    • Iodine deficiency
    • Enzymatic defects within thyroid gland
    • Thyroid hypoplasia
    • Maternal ingestion of goitrogens during fetal development
  • Secondary hypothyroidism may be due to:
    • Pituitary insufficiency because of pituitary tumors
    • Surgical therapy
    • External pituitary radiation
    • Postpartum pituitary necrosis
    • Trauma
    • Infiltrative processes (e.g., metastatic tumors, tuberculosis)

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  • Occurs in 1.5–2% of women and 0.2% of men.
  • Incidence increases with age.

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  • Postpartum status
  • Family history of autoimmune thyroid disorders
  • Previous head, neck, or thyroid irradiation or surgery
  • Autoimmune endocrine conditions, including:

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Signs and Symptoms

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  • Symptoms:
    • Dry skin
    • Cold intolerance
    • Weight gain
    • Constipation
    • Weakness
    • Lethargy
    • Fatigue
    • Muscle cramps
    • Myalgia
    • Stiffness
    • Loss of ambition or energy
  • Signs:
    • Coarse skin and hair
    • Cold or dry skin
    • Periorbital puffiness
    • Bradycardia
    • Slowed or hoarse speech
    • Proximal muscle weakness
    • Slow relaxation of deep tendon reflexes

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Laboratory Tests

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  • In primary hypothyroidism, thyroid-stimulating hormone (TSH) elevated.
  • In secondary hypothyroidism, TSH levels may be within or below reference range.
  • Free and/or total thyroxine (T4) and triiodothyronine (T3) serum concentrations low.
  • Antithyroid peroxidase antibodies and anti-TG antibodies often elevated in autoimmune thyroiditis.

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Differential Diagnosis

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  • Chronic fatigue syndrome
  • Depression
  • Congestive heart failure
  • Anemia due to other causes
  • Amyloidosis

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  • Restore thyroid hormone concentrations in tissue.
  • Provide symptomatic relief.
  • Prevent neurologic deficits in newborns and children.
  • Reverse biochemical abnormalities.

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  • Table 1. Thyroid preparations used for treatment of hypothyroidism.
  • Levothyroxine(T4) drug of choice for thyroid hormone replacement therapy.
    • Once product selected, therapeutic interchange discouraged.
    • Starting dose for young patients with longstanding disease and patients older than 45 years without known cardiac disease 50 mcg daily, increased to 100 mcg daily after 1 month.
    • Recommended initial dose for older patients or those with known cardiac disease 25 mcg/day, titrated upward in increments of 25 mcg monthly.
    • Individualize dose with appropriate TSH monitoring; average maintenance dose ~125 mcg/day.
    • Drug interactions
      • Oral absorption may be impaired by:
        • Cholestyramine
        • Calcium carbonate
        • Sucralfate
        • Aluminum hydroxide
        • Ferrous sulfate
        • Fiber supplements
      • Increased clearance may occur with:
        • Rifampin
        • Carbamazepine
        • Phenytoin
      • Amiodarone may block conversion of T4 to T3.
  • Thyroid, USP (desiccated thyroid)
    • Derived from animal glands and may be antigenic.
    • Inexpensive generic brands may not be bioequivalent.
  • Thyroglobulin
    • Purified hog gland ...

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