Understanding your thyroid function tests. In this presentation, we will talk about the free T4, freeT3, and TSH blood tests which are used by your providers to understand whether your thyroid gland is functioning in a normal fashion or whether an overactive or underactive gland may be present. When we talk about the thyroid, we actually talk about two glands. The thyroid itself is a bow tie or butterflyshaped gland that sits at the base of your neck. In addition, we also must talk about the pituitary gland. This is a small gland that is located in the base of the brain. The pituitary gland is important because it monitors and regulates the activity of the thyroid. It regulates the thyroid through the production of a hormone called thyroid stimulating hormone which we abbreviate TSH. As the thyroid stimulating hormone floats around in the bloodstream, it reaches the thyroid where it is necessary for the normal growth and maintenance of the gland. In addition, it also stimulates the gland to produce thyroid hormone. There are two main forms of thyroid hormone in your blood. One is called T4, the other is called T3. As the thyroid hormone is floating around in the blood, it passes back through the pituitary which determines whether the amount of thyroid hormone in the blood is correct for you or not. In response, it'll either put out more or less TSH in an attempt to regulate the amount of thyroid hormone which is being made. If you think about your pituitary gland like a thermostat and your thyroid like a furnace, the relationship between the pituitary and the thyroid follows this same model. Just as the thermostat senses the temperature of the air in your home and then signals the furnace to put out more heat if it's needed, your pituitary gland senses the level of thyroid hormone in the blood and then through thyroid stimulating hormone signals the thyroid to put out more hormone if it is required. Another way we can think about this relationship is to think about the thyroid hormone and the TSH being like children sitting at opposite ends of a teeter totter. When all of their values are normal, the hormones then will be in balance with one another. In hyperthyroidism, there is an excess production of thyroid hormone. This extra thyroid hormone reaching the pituitary causes it to diminish its production of TSH and an effort to decrease further thyroid hormone production. This is analogous to when you're driving your car. If you see that you're over the speed limit, you do not necessarily have to apply the brakes, but instead you can let your foot up off the gas to allow the vehicle to slow down. There is no break on the system between the pituitary and the thyroid. So the only thing the pituitary can do is to decrease its production of TSH in an effort to slow the production of thyroid hormone. We return to our teeter totter analogy, it makes sense that as the T4 andor the T3 levels increase that the TSH level would decrease. It is often confusing to patients when their providers tell them that their thyroid gland is overactive because the thyroid hormone level that they measured was low. In this situation, the provider is referring to the TSH. The TSH is typically the test that is used by providers in order to screen for thyroid disease. So, the finding of a low TSH value would be consistent with you're having an overactive thyroid gland. Now let's consider hypothyroidism. In this situation, there is a decrease in the production of thyroid hormone by the thyroid gland. In turn, that decreased production is perceived by the pituitary which responds by increasing the amount of TSH that it produces in an attempt to drive the thyroid to produce more hormone. If we return to our teeter totter analogy, we see that as the T4 and T3 levels decline that the TSH side of the teeter totter will rise. Again, when your doctor screened you for your thyroid condition, uh they may have commented to you that they thought the gland was underactive because of an elevation of the TSH. In summary, the T4 and T3 are the primary hormones that come out of the thyroid gland. The TSH is the hormone made by the pituitary that reflects the pituitary gland's opinion on the adequacy of the level of thyroid hormone in your blood. As we see in this teeter totter, when all of those values are normal, everything is in balance and the thyroid is functioning correctly. In hyperthyroidism, as the T4 and T3 end of the teeter totter go up, the TSH level declines. In hypothyroidism, the excess hormone causes the pituitary to decrease the production of TSH in an attempt to decrease the production of thyroid hormone by the thyroid gland. Finally, in hypothyroidism, as the level of T4 and T3 in the bloodstream go down, the level of TSH goes up. Again, the elevation of the TSH is an attempt by the pituitary gland to signal the thyroid to increase its production of thyroid hormone to make up for the low levels of thyroid hormone that it senses in the blood. I hope this brief overview of thyroid function tests has been helpful. If you have any questions, please feel free to contact our office at the number on the screen. Thank you very much.