If you take a look at the diagram below, you can see that the thyroid hormone production is a complex pathway and is not just an on/off switch.
There are numerous steps in the pathway and an imbalance or poorly functioning step on this pathway can lead to less than optimal thyroid function.
The thyroid produces 2 hormones: T4 and T3. About 94% is produced as T4 with the remaining being T3.
T4 has to be converted to T3 before it can be used by your body.
Not all the T4 produced by the thyroid gland is converted to T3. About 60% is converted to T3, 20% is converted to rT3. The remaining 20% is converted to other forms of T3 that can converted to a useful T3 form in the gut, but only if there is a healthy and balanced gut microbiome.
Most of the conversion of T4 to T3 takes place in the liver with some conversion also taking place in muscle, heart and nerve cells.