Many people do quite well taking T4 (levothyroixine, synthroid, levoxyl), but often people find that they do not feel normal in one way or another when given T4. Sometimes it's a matter of raising the dose and getting the TSH (thyroid stimulating hormone) into the correct range (see my last post). Another possibility is that one's body is not converting the T4 into T3 (tri-iodothyronine, cytomel). T3 is the thyroid hormone that controls or contributes to 80% of the thyroid activity in the body. So, when you take T4 or make it yourself, parts of your body (e.g., the liver) must be able to convert it into the most active thyroid hormone, T3. But sometimes (insulin resistance, stress, chronic illness) the body does NOT convert the T4 into T3. In that case, your T4 and even your TSH might be normal, but you are still not getting enough thyroid hormone.
For example, your T4 might be in the middle of the normal range, but when you look at the free T3, it might be in the lower part of the normal range, indicating poor conversion. If that is the case, your body temperatures might be low, and you might have other symptoms and signs of low thyroid function. In that case, after making sure that your hypothalamic-adrenal axis (HPA) and cardiovascular system can handle it, you might want to consider a combination of T4 and T3 (e.g., Levoxyl and Cytomel as separate prescriptions), or porcine thyroid hormone, which comes as a fixed combination. In my experience, many patients who have difficulty with conversion of T4 to T3, and who can handle it physiologically, do best with the porcine thyroid. Ask your doctor to check the TSH (best checked in the morning), along with the free T3 and Free T4. If the conversion is an issue, and you have signs and symptoms and can tolerate it, consider either adding some T3 to your T4, or using porcine thyroid hormone instead.