Understanding TSH, T4, and T3

It can be difficult to wrap your head around how the different hormones TSH, T4, and T3 function.  The TSH (thyroid stimulating hormone) is a pituitary hormone.  This stands for thyroid stimulating hormone.  It is a signal that comes from the pituitary gland to tell the thyroid that there is not enough active thyroid hormone in the cells and that it needs to kick some more thyroid hormone out.  The T4 (thyroxin) is the thyroid storage hormone.  The body in a perfect state of health has to convert the T4 hormone into the active thyroid hormone T3 (triiodothyronine).  The cells of the body receive this active T3 hormone through receptors.

One way I like to look at it is to correlate it to a light switch, a light fixture, and actual lighting that you have in front of your face to see an object.  The light switch is the TSH.  The light fixture is the T4.  The light in front of you is the T3.  When looking at thyroid function by means of a TSH test, you are merely looking at the light switch to see if it is turned in the on position.  When you measure the storage thyroid hormone T4 (Free T4 test), you are looking at the light fixture to make sure there is a light bulb in there.  When you measure the active thyroid hormone (Free T3 test), you are assessing whether or not you have light to see by.  If you don’t have enough light to see by, that would be referred to as having hypothyroid symptoms.

There are numerous reasons why the TSH is iffy in testing thyroid function.  In this post, I will only address a correlation as to determining dosage of thyroid treatment in regards to the “light source.”  Say that you determine that you have faulty lighting and you just don’t have enough light to see by, and so you provide an external source of lighting such as a skylight or other window.  This would be like taking thyroid hormone replacement.  If you have enough light to see by, you are not prompted to go flip the switch to turn the light bulb on.  The same kind of goes with thyroid replacement.  When the cells of the body are receiving the active thyroid hormone, the feedback loop has no need to tell the pituitary gland to tell the thyroid to produce more hormones.  Therefore, we see a drop in the TSH level.  Doctors are trained to perceive this is being hyperthyroid and are prompted to reduce the dosage, then throwing the body back into hypothyroidism.  When only TSH and Free T4 levels are tested, the main function of the thyroid is left out, the T3 levels.  This is the same as looking at a light switch or a light fixture to tell you whether you have enough light to see by.  Proper testing for a complete picture of thyroid function should contain the lab tests TSH, Free T4, Free T3, and a Reverse T3.  The ratio between Free T3 and Reverse T3 will help you to know how much active thyroid hormone T3 is getting received by the cells in your body.

Understanding TSH, T4, and T3        © Leslie Boswell 2014