Synthetic forms of the thyroid hormone T4 are generally called levothyroxine, and they are considered the standard treatment for hypothyroidism. Levothyroxine is considered a levoisomer, which simply means that the compound was produced by rotating at a specific wavelength but this has no impact on its biological function. As such, levothyroxine mimics the natural hormone. Though this hormone is man-made, synthetic T4 hormones are very similar to the T4 that is produced and released by the thyroid gland.
Types of T4 Supplements
There are currently six FDA-approved brands of synthetic T4 supplements available:
Synthroid has been the most commonly-prescribed brand of T4 for hypothyroidism but health insurers push to lower costs has required many patients to switch to the generic supplement, levothyroxine sodium. The T4 supplement delivers a steady, prolonged dose of the needed hormone. When making a change in the formulation of your supplement, having your levels checks and adjustments in dosage may be necessary to assure your TSH levels remain in the desired range.
All the approved brands of T4 are considered essentially the same, and most importantly, provide favorable hormonal effects. In other words, there is no significant difference in their composition. However, that does not mean that these brands are exactly the same. The bioavailability of a given brand at a given time after ingestion might be different. That's why much of the endocrinology community—the American Association of Clinical Endocrinologists, the Endocrine Society, and the American Thyroid Association—believe that once you start with a brand, you should stick with it, if at all possible.
Finding your ideal T4 dosage is essential. The right dosage keeps hypothyroidism from interfering with your life. The wrong dosage can make it an even bigger problem than it was before you sought treatment.
Getting the right dosage is important, but don't expect the dosage you start out with to be the dosage that you eventually stay with. Doctors often use weight as a guideline for determining dosage. Some clinicians use the formula of 1.6 micrograms of T4 for every 1 kilogram (or 2.2 pounds) of body weight for a starting dosage. Others prefer a more conservative approach, starting patients at a very low dose (perhaps as low as 25 micrograms). Note that the 1.6 micrograms is considered a full replacement dose. This means that if part of your thyroid still functions properly, you won't need this full dose because your body is continuing to make some of its own T4, in addition to the hormone supplied by the pill.
Because it's common for dosages to change at the start of treatment, your doctor will likely monitor your thyroid stimulating hormone (TSH) levels after two or three months (though some doctors may check as soon as four weeks) from your first day of treatment. And since hormone replacement therapy is usually a lifelong treatment, you should get checked every year to make sure you're taking the right dose if you're on a stable dose. You should communicate with your doctor more frequently if your dose is being adjusted.
Doctors often err on the side of caution when prescribing starting dosages of T4 for a variety of reasons. For one, starting at a low dose and moving up lets your heart get used to the increased metabolism. Also, your doctor doesn't want to risk inducing hyperthyroidism—a condition caused by high levels of thyroid hormones. You can learn more about the symptoms of over-treatment below.
Even with a moderate dose of T4, some patients are susceptible to over-treatment symptoms. Elderly patients with weaker hearts and people with heart arrhythmias (irregular heartbeat) are especially sensitive to thyroid hormone. Generally, doctors like to start with a slightly lower dose in these patients, in order to avoid or worsen irregular heartbeats.
Below are some of the symptoms of over-treatment:
If you experience any of these symptoms throughout the duration of your hypothyroidism treatment, talk to your doctor immediately. He or she will first check your blood tests, before deciding on a dose.
Taking Other Medications with T4
Because T4 is ingested, it must be absorbed from the gastrointestinal tract into the bloodstream. The medications and supplements listed below should NOT be taken at the same time as T4, as they can interfere with your body's ability to absorb the hormone:
If you need to take any of the medications above, you should take them three or four hours before or after you take T4. Your doctor will recommend the best time to take T4 (common times to take the medication are first thing in the morning or right before you go to bed, when you have an empty stomach).
Within two weeks of taking synthetic T4 hormone supplements, you'll begin to feel the effects of the treatment. When taken as directed—and with the right dose—you'll find that T4 hormone replacement therapy can effectively manage your hypothyroid symptoms.