In one study of patients who had Hashimoto's disease, but whose TSH thyroid levels were normal ("euthyroid"), one group of patients received levothyroxine treatment, and the other group did not receive treatment.
After 15 months, the treated group had significantly increased Free T4, significantly decreased TSH levels, and a reduction in both anti-thyroglobulin antibody (Tg-Ab) and anti-thyroid peroxidase antibody levels (TPO-Ab). The actual size of the thyroid also decreased in the treated group, while those not receiving treatment had an increase in the size of their thyroid.
The researchers reported that although levothyroxine treatment is "mandatory in hypothyroid autoimmune thyroiditis patients, LT4 treatment which is shown to inhibit autoimmune process in animal models is still controversial in euthyroid Hashimoto's disease patients where disease has not destroyed the thyroid gland enough to cause hypothyroidism."
They found, however, that LT4 treatment at doses keeping TSH at low-normal levels appear sto be effective not only in decreasing the autoantibody levels but also in the goiter size, which could ultimately prevent progression to overt autoimmune hypothyroidism.
Source: Duygu Yazgan Aksoy, et. al. "Effects of Prophylactic Thyroid Hormone Replacement in Euthyroid Hashimoto's Thyroiditis" Endocr J (Japan) Vol. 52: 337-343, (2005).
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Mary Shomon, About.com's Thyroid Guide since 1997, is a nationally-known patient advocate and best-selling author of 10 books on health, including "The Thyroid Diet: Manage Your Metabolism for Lasting Weight Loss," "Living Well With Hypothyroidism: What Your Doctor Doesn't Tell You...That You Need to Know," "Living Well With Graves' Disease and Hyperthyroidism," "Living Well With Autoimmune Disease," "Living Well With Chronic Fatigue Syndrome and Fibromyalgia," and the "Thyroid Guide to Fertility, Pregnancy and Breastfeeding Success." Click here for more information on Mary Shomon.