This subject is so complex.... I just had a formal CE on it in fact.

Unless a person is on high dose replacement hormone....there will be residual activity from the thyroid gland.

The feedback loop will depress the thyroid depending on DOSE.
Patients with glands completely removed will not be making T3.
However patients like me who only need 75mcg a day, will be making some endogenous hormone above the 75mcg coverage.

Because the dessicated products contain T3, the half life for this active form is very short, so timing is more specific.

For LEVOthyroxine like Synthroid and others, timing is less critical because the T4 has a 1/2 life of DAYS. Hence once you are on it for a while, you can even skip a day here and there if you run out, or lose them etc, without incident. If you are one or two hours early or late taking, that is insignificant since there is T4 hanging around for DAYS...as a backup.

In the case of Doody's question, her post #8 says, her tests do not show indication of Hashimoto's. So in HER situation, having low T4, in the face of "normal" TSH may indicate a lack of iodine in the body to make the T4. Iodine is conserved by the body to some extent and if something is wrong there and it is LOST and cannot be recycled to make more hormone, it needs to be supplemented.

There is a large range of patients with hypo symptoms who are clinically hypo or subclinically hypo, all the way to those with removed glands. Treatments vary therefore for the various situations.

One factor to always consider is that too much thyroid hormone given, will be adjusted by the body's feedback loop to slow production in the gland. This is dose dependent. In my case my doses were adjusted to reduce a goiter. Once that was done, my hypo symptoms abated. But as doses increase more and more gland stops producing both T4 and T3. Once that becomes total, then taking a T3 product becomes more important.

In the elderly medical suggestions are to keep doses within certain limits, because thyroid hormone over time accelerates bone loss. It enables osteoclasts which tear down bone. So doctors will balance things to avoid excessive bone loss.

The doses of T4 synthetic like Synthroid are in micrograms.
The doses of dessicated are in milligrams. They are not easily interchangeable. for example 100mcg of Synthroid is not equal to 100mg of dessicated as a rule.
http://melissa-murfin.suite101.com/c...roxine-a146765