There is a lot of new research that suggests taking t3 & t4 together--Most doctors prescribe T4 (Synthroid) to treat an under active thyroid. T4, though, is fairly inactive until the body converts it into T3, or activated thyroid hormone. If the problem is only with the thyroid gland itself, prescribing Synthroid will work just fine. However, during periods when the body wants to conserve energy (for example, during times of infection or with CFS/FMS), the body slows down its metabolism. It does this by decreasing the production of active T3 from T4, which is turned into inactive "reverse T3" instead. In some cases, the body may get "stuck," and becomes unable to make adequate T3. Because of this problem, many physicians prefer to use compounded or Armour Thyroid, which contains a mix of T4 and T3.
If you suffer from chronic fatigue and have achy muscles and joints, heavy periods, constipation, easy weight gain, cold intolerance, dry skin, thin hair, a change in your ankle reflexes called a delayed relaxation of the deep tendon reflex (DTR), or a body temperature that tends to be on the low side of normal, you should consider asking your doctor to prescribe a low dose of Armour thyroid hormone. As long as you do not have underlying heart disease and you follow up with a blood test to make sure that your Free T4 thyroid levels are in a safe range (going above the upper limit of normal may aggravate osteoporosis, a problem already common in CFS/FMS) a trial of low-dose thyroid hormone treatment is usually quite safe and may be dramatically beneficial.
The new new version of Armour works great (the new version sucked--old was great but with the mad cow scare everyone got nervous)