Ive told you for years that the TSH (thyroid stimulating hormone) test is not a good way to detect low thyroid. Your pituitary makes TSH. Theoretically, if your thyroid is low, TSH will increase to kick your thyroid into action. But TSH tests are not very accurate. And a new study now supports my contention.
This study followed 85 subjects with hypothyroidism. All of them were taking T4 (i.e., Synthroid or levothyroxine). The researchers had 114 controls with normal thyroid levels. The researchers found that even though TSH was normalized in those taking T4, their T3 levels were lower. That means they were deficient in overall thyroid activity.
The level of T3 in your pituitary regulates TSH. So it may not be the best indicator of adequate thyroid activity in all tissues of your body. T4 is the major hormone produced by your thyroid. It also makes appreciable T3. The difference between the two is simply that T4 has one more iodine (a total of 4) atom attached to it than T3 (which has 3). T3 is far more active than T4. Your thyroid makes some T3, but your tissues (liver) must convert T4 to T3 for the most thyroid activity. As you age, this process slows down.
T3 ought to be provided along with T4. The regular commercial preparations used by the overwhelming majority of doctors in the U.S. do not have any T3. Without it, you could easily be deficient in thyroid activity even with normal levels of TSH and T4.
Action to take: If you are taking only T4 and not satisfied with the results, please take this article to your physician and ask for a desiccated thyroid preparation. These include Armour thyroid or Nature-thyroid. Generally, I replace 0.1 mg of T4 with 1 grain of desiccated thyroid. If you are taking more or less T4, simply adjust the desiccated thyroid proportionately. In my experience, 90% of people prefer desiccated thyroid. About 10% or less will prefer the commercial Pharma T4.
Ref: Wien Klin Wochenschr, 2006, 117(18): 636-40.