Etiology and Management of Hypothyroidism
May 2011 – by F. Ling, MD
More common in women than in men, hypothyroidism occurs in up to 5% of the population with another 3% or considered subclinical. Worldwide, the most common cause is iodine deficiency. In the U.S., the most common cause is the auto-immune condition of Hashimoto's thyroiditis in which a goiter is present and the gland is unable to produce thyroid hormones. Loss of thyroid function can be related to previous surgery or treatment of radioactive iodide for hyperthyroidism (Graves' disease). Lymphocytic thyroiditis is another potential cause of hypothyroidism. This condition should be strongly considered if a period of hyperthyroidism is followed by symptoms of hypothyroidism in postpartum women. Pituitary dysfunction (secondary hypothyroidism) and hypothalamic defects (tertiary hypothyroidism) are also potential causes of hypothyroidism. An often-overlooked cause is medication. These include medications used to treat hyperthyroidism. These include methimazole [tapazole] and propylthiouracil [PTU], lithium, potassium iodide, and even Lugol's solution.
Once the correct diagnosis is made, treatment of hypothyroidism is typically expected to be life-long. The most common form of thyroid replacement is synthetic levothyroxine. Its once-daily dosing makes it convenient to take, and it usually provides a predictable response. In young, healthy women, the starting dose can approximate what the final dose will be, i.e. approximately 100 mcg daily. In older patients, a gradual increase in dose may be needed to avoid palpitations, chest pain, or even myocardial infarction. Initially, levels of TSH are drawn approximately every six weeks to determine the correct dose of levothyroxine. Once the TSH is within normal limits and the patient's symptoms are addressed, the TSH can be redrawn annually or as needed based on new symptoms.