Putting it all together.
TSH is produced in the brain. It stimulates the thyroid to produce T4. The T4 is converted into T3 and reverse T3 which bind to cell receptors and start activities in each cell of the body.
When measuring thyroid hormone levels, we can detect them in the blood with lab tests. When looking at T4 and T3 we can measure total and free levels. The total levels are all the T4 or T3 in the blood. Some of this is bound to carrier proteins and some is free. The free, unbound hormone is the active, available hormone in the blood to do the work. Therefore, we want to know what the Free T4 and Free T3 levels are.
Let's work backwards. The thyroid hormone with the most activity on the cells of the body is the T3. Therefore, we want to know what the levels of T3 are in the blood and mostly what the level of Free T3 is. Because if the T3 is low, then thyroid affects on the cells will be low. I generally like to see the level of Free T3 at 3.0 and above. This seems like a good number for a healthy adult. If you look at healthy children, their Free T3 levels are at 3.5-4.5.
The next hormone to look at is the T4. I consider it a storage hormone since it has little activity on the cells. It has to be converted into T3 to have activity. But, if you aren't making enough T4, then you won't have enough T3.
Reverse T3 is the red herring in the mix. It usually measures about 8-11. It is always around. When it gets too high is when issues arise. It has been shown in chronically ill and severely ill patients in intensive care units that Reverse T3 levels rise dramatically. This Reverse T3 binds to the cell receptor for the real T3 and blocks the real T3 from binding and doing its job. Since it only has 1/10 the activity, it doesn't have the same stimulation affect on the cells and produces a defacto low thyroid condition.
TSH is the hormone produced in the brain that travels to the thyroid gland through the blood to stimulate production of T4. This is a favorite hormone to measure. But there is a problem with only measuring TSH. Studies have shown that the cells of the brain are more sensitive to T3 and T4 compared to the cells of the rest of the body. In other words, the cells of the brain are happier with lower levels of thyroid hormones. This means that they stop sending out TSH when total blood levels of T4 and T3 are lower than what the body cells needs. In a bucket analogy, the brain is happy with the bucket half full of thyroid hormones and the rest of the body needs the bucket filled to the top. This results in lower overall circulating thyroid levels.
Interpreting the Results.
When first measuring thyroid functions, I like to look at all of the above hormones. I want to know the free and total levels of T3 and T4.
A few examples are below:
I want to know the levels of T3. Because if someone has low free T3, then they have low thyroid.
I want to know the Reverse T3 level because if someone has high Reverse T3 then a chronic issue is going on that needs to be addressed. A person with normal Free T3 levels and high Reverse T3 levels will feel low thyroid because of the 1/10 activity factor of the Reverse T3 blocking the cell thyroid receptor.
I want to know the Free T4 level because if someone has normal T4 levels and low Free T3, then they are not converting the T4 into T3 and are going feel low thyroid.
Finally, I use the TSH level to confirm if someone has low thyroid. The TSH will be high if the person has low thyroid. Because of the brain being more sensitive to T4 and T3, I don't find TSH alone helpful beyond if it is too high.
Knowing the above measurements, allows the tailoring of thyroid medicine to the specific patient for the best results.
Also, the above has to be in context of how the person feels. If they feel totally normal, then the lab numbers aren't an absolute for treatment.
Here is a link to a lecture I gave at the San Diego Osteopathic Medical Association on Subclinical Hypothyroidism which has a lot of the above information in it.
As always, if the situation becomes overwhelming, seek professional help regarding your physical and mental symptoms.