Let's hear you pronounce it: triiodothyronine. There, now you know why the name is usually shortened to T3. T4 (thyroxine) and T3 are the main thyroid hormones. T3 is five to eight times as strong as T4 (taking into consideration that it's absorbed at a higher rate than T4), and it's biologically more active. T4 is like the food in your refrigerator, while T3 is like the food on your plate.

T4 is slow acting, with a half-life of about one week—after a week, you have about half the level of the T4 still in your body, a week or so later you have half of that half remaining, and so on. Its full effects aren't reached until about six weeks after starting or changing a dose, which is why lab tests are optimally done every six weeks or so until a hypothyroid patient has reached satisfactory and stable thyroid hormone levels. T3, on the other hand, has a half-life of about a day. Hypothyroid people on T3 sometimes feel its effects within minutes after taking it.

T3 is available as a separate synthetic medication with the brand name Cytomel in the US and Canada, and Tertroxin in the UK. It's usually prescribed along with a synthetic T4 medication. T3 is also part of some combination T4/T3 drugs. In the US, Thyrolar is a synthetic T4/T3 combination. Natural, desiccated thyroid from pigs' thyroids with the brand name Armour is sold in the US, and in Canada, desiccated thyroid without a brand name is made by Parke-Davis.

A thyroid gland that functions normally produces T4 and T3. Twenty percent of the T3 circulating in the body comes directly from the thyroid gland, and the remaining 80 percent comes from conversion of T4. Because of this conversion process, most doctors prescribe only synthetic T4 medication (Synthroid, Levoxyl, Levothyroxine, Levothroid, Eltroxin, Unithroid, and others). Many hypothyroid patients do fine on T4 only.

However, many others don't, and they need T3 supplementation in addition to T4. If the thyroid gland is malfunctioning and not producing enough—or any—T4, why assume that it still puts out enough T3, or that the body converts enough of its T4 to T3?

The addition of T3 often helps with many symptoms of hypothyroidism that may not disappear with supplemental T4 only. It has improved people's libido, memories, and vision. It has eliminated or greatly reduced brain fog, feeling cold, constipation, depression, chronic fatigue, headaches, insomnia, muscle and joint pain, and chronic sinus infections. For some people, but not all, it has helped them finally lose weight. A small percentage of people who try it feel worse or no better on it.

Do lab tests show if a person needs T3 supplementation? Sometimes. If the free T3 is lower in its range than the free T4 is, this suggests that more T3 would be beneficial for that person. On the other hand, some people who have posted in alt.support.thyroid have had lab results that did not indicate a problem with T3, but they still had symptoms of hypothyroidism, and the addition of T3 helped them.

Lab results do not tell the whole story. However, most people with hypothyroidism in alt.support.thyroid feel best when their free T4 and free T3 levels are in the upper part of their ranges. The exception is with people who are on desiccated thyroid. Because it contains a higher ratio of T3 to T4 than our thyroids produce, people taking it have a free T4 level that's lower in its range when the free T3 level is where it should be, in the upper part of its range. Total T4 and T3 tests can help paint a broader picture of what's happening.

See the links at the right on this page. The "T3 References" file starts with medical journal references because they are the most important to doctors, but if this subject is new to you, you'll probably find the articles in the other sections easier to digest first. The above list of symptoms that T3 has helped with is from the "T3: Patients' Experiences" compilation.

If you want to discuss T3 supplementation with your doctor, we recommend that you read through the files here, and perhaps print copies of them for your doctor. Because T3 is so much stronger and faster acting than T4, it's important to get the doses right. On pages 285 and 286 of his book The Thyroid Solution, Dr. Ridha Arem describes how to adjust T4 doses when supplementing T4 with T3. Some people in our group have brought this book to their doctors, and their doctors have used this protocol to successfully add T3 to the medication mix.

Kevin G. Rhoads discusses TSH, T3, and T4 in more detail in his "Thyroid 101 and Basic Fallacies" post. See also the related article, "The TSH Story."

Lois Summers

"T3 References" is a compilation of references about T3 supplementation from medical journals and organizations, doctors' websites, interviews with doctors, other websites, and books. They all indicate that T3 in addition to T4 is beneficial for many patients with hypothyroidism. Except for in the book section, links to the full texts are provided. Printer-friendly versions are available.