Thyroid Disease : Determining Your Medication Dose

Excerpted from Thyroid Disease: Understanding hypothyroidismand hyperthyroidism, Copyright ©2012, Harvard University.

Your initial dose of thyroid medication is carefully selected based on your weight, your age and whether you have any other medical conditions. A general guideline is 0.8 mcg for every pound of weight (or 1.6 mcg for every kilogram of weight). But your doctor may decide on a lower or higher dose depending on your individual circumstances.

Treatment of an older person begins gradually because of a higher risk of underlying heart disease. In older people, achieving full hormone replacement too quickly may put stress on the heart or central nervous system. If medication increases slowly, the heart and central nervous system progressively adjust to a faster pace. For instance, your doctor may start you with 12.5 to 25 mcg of synthetic T4 (levothyroxine sodium) per day and increase the dose every four to six weeks until laboratory tests show that your TSH and T4 levels are within the normal range. Doctors will prescribe an even more gradual dosing schedule for certain patients who experience symptoms of heart failure, angina or anxiety.

The dose is also based on the severity of your hypothyroidism. For instance, someone with autoimmune-induced hypothyroidism whose thyroid is still producing some hormone may require only partial hormone replacement. But a person whose thyroid has been removed requires total hormone replacement, which on average means a higher dose.

Another important factor in determining dose is whether you are on any medication that may interfere with the absorption or metabolism of your thyroid medication. For instance, if you are on estrogen therapy, birth control pills or certain antidepressants, you may need a higher dose (see “Drug Interactions graphic,” page 23). So it’s important to tell your doctor if you are taking any other medication, or if you begin to take another medication after you start taking your thyroid pills.

Think of your initial dose as an estimate of your need. There is likely to be a bit of trial and error, because your thyroid hormone needs are very precise, and you probably will need adjustments until your TSH level is within the normal range. Your doctor may begin with a low dose because too much thyroid hormone may cause symptoms of hyperthyroidism, such as nervousness, anxiety or racing heart. If you experience any of these symptoms, contact your doctor immediately.

Once the initial dose has been decided, remember that it may take several weeks before you experience any changes in your system. T4 is a slow-acting hormone, and you are not likely to feel its effects immediately. Generally, anywhere from four to eight weeks after you begin taking your medication, your doctor will want to check your TSH level. If, at that point, your TSH level still is not within the normal range, the doctor will adjust the dose, repeating this process until your TSH level is normal and your symptoms have improved. Once the appropriate dose has been established, you will have a TSH test, and sometimes a test for free T4, every six to 12 months.

Dr. Jeffrey R. Garber is an Associate Professor of Medicine at Harvard Medical School, chief of the endocrine division of Harvard Vanguard Medical Associates, and President of the American College of Endocrinology (ACE). His book, The Harvard Medical School Guide to Overcoming Thyroid Problems, and monograph Thyroid Disease: Understanding hypothyroidism and hyperthyroidism, Copyright ©2012, Harvard University, were written for members of the lay public interested in learning about thyroid disorders.