When T4 is not Enough, It May Be in Your Genes

Even though it is considered standard treatment, not everyone with hypothyroidism does well on T4 (Synthroid, or levothyroxine) alone. Some people complain that they just don’t feel right on T4, the inactive form of thyroid hormone. They are still tired, or sluggish, or have “brain fog” or other vague symptoms. Or they are gaining weight. Just about every doctor who treats thyroid problems has seen this regularly.

Unfortunately, it is often the patient who has to convince the doctor that T4 alone is not working. And it is often the patient who suggests that T3, the active form of thyroid hormone, may be what’s needed. One reason doctors don’t think to use T3 is that most of the few studies that look at T4 vs. T3 therapy see no advantage to using T3. (A problem with these studies is that they did not dose the T3 to normalize body temperatures, which is key). One study, however, did find that there was significant psychological impairment in people taking T4 alone, compared to a control group of people with normal thyroid function.

Some researchers are now speculating that the people who don’t respond adequately to T4 may have a variation in a gene that produces an enzyme, type 2 deiodinase (D2.) This enzyme converts T4, the inactivate form of thyroid hormone, to T3, the active form. People with this gene may have D2 that does not convert T4 to T3 as effectively, which would explain why they may not respond well to T4.

Just how many people may have this genetic variation is not known. (In one study, 16% of people had the variation). Are you one of these people? The genetic testing for this is not readily available.

But there is an easier way to find out if you need more than T4. Check your body temperature. If it is consistently low—below 98.5 F., or 36.94 C, but typically lower than 97.8 F, or 36.56 C.— there’s a good chance that you have a slow metabolism. It may mean that you are not converting enough T4 to T3. It may also mean that taking more T4 is not going to help you, because levels of D2 can be suppressed as blood levels of T4 rise. People with low body temperatures often do better if their treatment is T3 alone, not a combination of T4 and T3.

Share your thoughts and concerns with your doctor, who can call us at 800.420.5801 to get more information about how to use T3, along with nutritional and herbs to support thyroid health, and to discuss your individual case. You can also use our website to find the health care practitioner closest to you who is trained in T3 treatment.

REFERENCES

Kim BW, Bianco AC. For some, L-thyroxine replacement might not be enough: a genetic rationale. J Clin Endocrinol Metab. 2009 May;94(5):1521-3.

Panicker V, Saravanan P, Vaidya B, et al. Common variation in the DIO2 gene predicts baseline psychological well-being and response to combination thyroxine plus triiodothyronine therapy in hypothyroid patients. J Clin Endocrinol Metab. 2009 May;94(5):1623-9.

Saravanan P, Chau WF, Roberts N, et al. Psychological well-being in patients on ‘adequate’ doses of l-thyroxine: results of a large, controlled community-based questionnaire study. Clin Endocrinol (Oxf). 2002 Nov;57(5):577-85.