As with all of the pituitary axises, the thyroid one is a little confusing when people talk about it but really quite simple when it’s drawn out.
Hypothalamus -> Pituitary -> TSH -> Thyroid -> T4 & T3
Free T4 is what normally gets measured to determine if someone is hypo- or hyperthyroid, but if you are just doing a basic screening test, you only need to measure TSH.
In the image, black represents the gland not functioning, while red is it going wild and crazy. remember that primary is the END ORGAND (in this case the thyroid).
Things in flux simply means that you’ve measured someone’s levels while things were in the midst of a change. This is because T3 and T4 levels can change fairly quickly, but it takes a while for the pituitary to catch up. This can happen either in the case of one of the glands ceasing to function or the person has recently started medication to correct thyroid hormone levels and the thyroid has changed but the pituitary hasn’t.