Hypothyroidism is much more common in women than men. It occurs in up to 5% of the population, with approximately 0.1-2% being overt and 4.3% subclinical. Hypothyroidism is more common in older adults and may be present in 5-15% of women over the age of 65. Worldwide, the most common cause is iodine deficiency. In the US, the most common cause is Hashimoto’s thyroiditis, an auto-immune condition in which a goiter is present and the gland is unable to produce sufficient thyroid hormone. Hypothyroidism can also be caused by previous surgery or treatment with radioactive iodide for hyperthyroidism (Graves’ disease). A transient postpartum thyroiditis can result in hyperthyroidism followed by transient hypothyroidism, or only hypothyroidism, lasting from several weeks to a few months. Pituitary dysfunction (secondary hypothyroidism) and hypothalamic defects (tertiary hypothyroidism) are also potential causes. Medications to treat hyperthyroidism such as methimazole and propylthiouracil (PTU), lithium, potassium iodide, and even Lugol’s solution can also cause hypothyroidism.
Primary hypothyroidism is diagnosed by a high serum TSH with a low serum free thyroxine (T4) concentration, while subclinical hypothyroidism is defined as a normal free T4 in the presence of a high TSH. Secondary hypothyroidism is diagnosed with a low serum T4 and a serum TSH that is not proportionately elevated. Common symptoms include fatigue, cold intolerance, weight gain, constipation, and menstrual irregularities. Findings may include bradycardia, cool or dry skin, hair loss, brittle nails, and a delayed relaxation phase of deep tendon reflexes.
Treatment of hypothyroidism is typically life-long, as autoimmune thyroiditis is usually permanent. Transient postpartum thyroiditis may not cause symptoms or require replacement. The most common form of thyroid replacement is synthetic levothyroxine. Its long half-life and once-daily dosing make it convenient to take. Generic formulations are acceptable and switching among formulations has not been shown to result in clinical problems. In young, healthy patients, the starting dose can approximate what the anticipated final dose will be with the goal being restoration of the euthyroid state and normalization of TSH. In older patients, a gradual increase in dose may be needed to avoid palpitations, chest pain, or rarely 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 resolve, the TSH can be rechecked annually or as needed based on new symptoms.
LeFevre ML. U.S. Preventive Services Task Force; Screening for Thyroid Dysfunction: Recommendation Statement. Ann Intern Med. 2015 May 5;162(9):641-50. doi: 10.7326/M15-0483.
Original Approval May 2011; Revised April 2015, September 2016; Reaffirmed March 2018.; Reaffirmed September 2019.
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