Thyroid Hormone Replacement
The generic name for the medication you are taking is levothyroxine sodium. It is identical to the substance your thyroid gland produces; in fact, the test that measures the level of thyroid in your blood can't tell what comes from your thyroid and what comes from the pharmacy.
Most people who take thyroid replacement need to take it life-long. The dose must be regulated based on how you feel, findings on your physical examination, and especially the results of a blood TSH test. "TSH" stands for thyroid stimulating hormone, which is produced by your pituitary gland. The blood level of TSH indicates where your thyroid level should be. A high TSH means your thyroid level is too low (and your dose should be increased); a low TSH means your thyroid level is too high (and your dose should be decreased). The ideal dose for you will result in a TSH level in the lower range of normal. If you have pituitary disease, TSH levels are unreliable. So, blood levels of free T4 are checked to determine the adequacy of your dose. High levels indicate that the dose of your medication is excessive, while low levels indicate that the dose is insufficient. Some patients may require adjustments in their dose even though their free T4 level is normal.
A thyroid dose that is too high can cause symptoms of hyperthyroidism (an overactive thyroid). These include palpitations (rapid, forceful heartbeat), nervousness, shakiness, irritability, diarrhea, weight loss, heat intolerance, and fatigue. A dose that is too low can cause symptoms of hypothyroidism (an underactive thyroid). These include sluggishness, constipation, muscle cramps, cold intolerance, weight gain, and fatigue. However, the dose can be off without causing obvious symptoms. That is why we recommend periodic blood tests.
Generally, you should have your TSH or free T4 levels checked 6 to 8 weeks after beginning treatment and 6 to 8 weeks after any dose change. Once an ideal dose has been confirmed by finding a normal level, the frequency of testing can be reduced. However, even if you have a normal level and you are feeling well, having the blood tests yearly is advisable.
You can take your thyroid medication whenever you want -- in the morning or at night, with food or on an empty stomach. We suggest you follow the same routine from day to day (be consistent). Your pharmacist may put a label on the prescription bottle advising you to take the medication on an empty stomach, but that is not necessary. If you prefer to take the medication with food, we can easily adjust your dose to take that into account.
There are several good brands of levothyroxine sodium (Levothroid®, Levoxyl®, Synthroid®). Because there can be differences between preparations, we advise staying with the same brand. If we forget to write "brand necessary" on your prescription, ask why. If your pharmacist gives you a different brand from what you were taking, let us know right away. It is possible to change from one brand to another, but we usually have to start the regulation process over again.
The medication is long acting -- that's why we wait 6 to 8 weeks to check the blood tests after changing a dose. If you miss a dose occasionally, it shouldn't make any difference. You won't notice anything different about the way you feel unless you miss several consecutive doses. If you know you have missed a dose, you can make it up by taking two tablets on one day.
There can be changes in dose requirements for no apparent reason, but these are usually minor. Circumstances that often require dose adjustment include weight change (gain or loss), pregnancy (a 50% increase in dose is often necessary), and sometimes with estrogen therapy or birth control pills.
Some medications will interfere with the absorption of thyroid hormone (and vice-versa) or otherwise affect the dose. These include medications to treat seizures, some stomach medications (antacids that contain aluminum such as Maalox¨ and Mylanta¨ and a prescription stomach medication called sucralfate [Carafate¨]), and iron. The problem can be avoided by taking the medications at different times of the day (for example, taking thyroid in the morning and iron at night). Cholestyramine (Questran¨), a medication used to treat high cholesterol, has somewhat more complicated effects on thyroid balance -- in the unlikely event this medication is prescribed for you, be sure to let us know before you start taking it.
Many over-the-counter cold and sinus medications have a warning on the label: "If you have a thyroid problem, do not take this medication unless you check with your physician." This warning is for patients with hyperthyroidism (an overactive thyroid) and does not apply to you.
Many thanks to Nelson B. Watts M.D. who provided this treatise on thyroid hormone replacement.
