Hypothyroidism is defined as a deficiency of thyroid hormone and is a common medical condition occurring in as many as one to two percent of the adult populations, up to five percent of people over age 60 and five to 15 percent of women over age 65.  The thyroid gland is found just beneath the skin in the lower, forward part of the neck.  Its purpose is to make thyroid hormones.  Thyroid hormones serve as a regulator of metabolic function throughout the body.  Hypothyroidism occurs in both men and women, but it is about five times more common in women than men.

According to the U.S. Preventive Services Task Force, a blood test to screen for hypothyroidism is not a standard part of routine blood work in adults, but there are circumstances that should lead to doing a test for hypothyroidism to rule out this disease. The symptoms can be non-specific, but if a patient is having symptoms that may be thyroid related, a test to rule out thyroid disease is indicated.

Some of the symptoms and signs that should prompt a thyroid blood test to look for hypothyroidism include weakness, fatigue, cold intolerance, constipation, weight change, depression, menstrual irregularity, dry skin, decreased heart rate (bradycardia) and anemia. There are other symptoms and signs that may be low thyroid hormone related, but these are the most common.

The single best test to screen for hypothyroidism is called TSH which stands for thyroid stimulating hormone. TSH is a hormonal signal produced by the pituitary gland (a hormone producing gland in the brain). TSH stimulates the thyroid gland in the neck to make thyroid hormone. If the thyroid gland cannot make adequate thyroid hormone for the body's needs, TSH goes up as the pituitary gland in the brain tries harder to stimulate thyroid hormone production even when the thyroid gland can't produce more. Thus, an abnormally high TSH is an indication of low thyroid function. The most common type of hypothyroidism is a lack of thyroid hormone production within the thyroid itself. This is called primary hypothyroidism. In rarer circumstances, there may be injury or alteration of pituitary function where TSH can be low or inappropriately normal leading to overall hypothyroidism.  This is called central or secondary hypothyroidism when the signal from the brain is the issue.  Futher discussion of secondary hypothyroidism is beyond the scope of this article.

Once hypothyroidism is diagnosed, and confirmed on repeat thyroid testing, the next topic of discussion is treatment. Primary hypothyroidism is treated with synthetic thyroid hormone called levothyroxine. The goals of therapy include improvement in symptoms, normalization of TSH level, reduction of the size of goiter (if the patient has enlarged thyroid as part of the presentation), and avoidance of overtreatment, which can have its own ill effects from too much thyroid hormone (called thyrotoxicosis). The initial dose of levothyroxine may vary depending on the initial TSH level and other factors such as body size. Your clinician has formulas to follow to calculate the initial dose. Sometimes older patients with cardiac disease will receive a smaller dose of levothyroxine than may be otherwise indicated with more gradual adjustment over time to avoid overtreatment, which could have cardiac effects. Dose may also vary if the clinician decides that a full replacement dose is not needed after interpreting the initial lab values. 

It takes approximately six weeks before a steady state of thyroid hormone levels is achieved. In most cases thyroid bloodwork is checked every six to eight weeks until a normal value of TSH is reached. Thyroid testing may then be done less often.

There are some important facts as to how to take levothyroxine. Levothyroxine should be taken on an empty stomach with water, either 30 - 60 minutes before breakfast or at bedtime several hours after the last meal. If a patient is on a calcium supplement, calcium should be taken with food, not at the time of taking levothyroxine. If a patient is taking iron in any amount, whether the iron is in an iron pill or contained in a vitamin pill, the iron should be taken at least four hours separated from the time levothyroxine is taken. Patients should also be cautioned not to take over the counter supplements or products labeled as "thyroid support," since these products are often high in iodine or may in fact contain some amount of thyroid hormone.  Do not take these over the counter products as they may interfere with thyroid measurements while on levothyroxine.

Treatment with either levothyroxine brand name or generic is acceptable. So long as the levothyroxine product used is consistent with the same manufacturer, either can be used. Problems occur when there is a change of generic products between manufacturers as this can lead to a change in the amount of levothyroxine needed to maintain steady thyroid levels.  Some clinicians insist on using a brand name of levothyroxine to maintain consistency in thyroid testing, but most patients should be able to use a generic levothyroxine as long as the manufacturer of the generic levothyroxine is consistent.

Another common question concerns the need for combination therapy of levothyroxine (which is T4) along with T3 (which is the active thyroid hormone produced from T4). Normally T4 is converted to T3 with the conversion happening in the body outside the thyroid gland.  For the great majority of thyroid patients, combination therapy with both T4 and T3 is not recommended. Nearly all patients can achieve a normal thyroid (euthyroid) state with levothyroxine (T4) therapy alone. In a review of nine studies on this topic, only one study reported beneficial effects of combination T4-T3 therapy on mood and quality of life.  Although T4-T3 combination therapy is not recommended routinely, there may some special circumstances where it may be considered. Patients who have not felt well since surgical removal of the thyroid, or intentional destruction of the thyroid with radiation, may benefit. Patients with a T3 measurement below or at the lower end of the T3 normal range may also benefit.

Hypothyroidism is a common condition. Screening for hypothyroidism should be done for patients with symptoms that are suggestive of the condition. The treatment of primary hypothyroidism is usually quite straightforward. Levothyroxine (T4) therapy is usually adjusted every six to eight weeks until normal thyroid levels are achieved. Hopefully, this information on hypothyroidism in adults will be useful to those adult patients with the condition.