Breaking down the thyroid panel - Part 1
One request we get repeatedly is for a “complete thyroid panel” instead of just a TSH. But what does this mean exactly? When one understands what a thyroid panel is comprised of, it becomes clear which components are useful - and which ones are not clinically significant, or even misleading.
TSH (thyroid stimulating hormone) - this hormone is released from the pituitary gland and regulates how much thyroid hormone (or T4) is produced from the thyroid gland. The TSH has an inverse relationship with T4. The higher the TSH, the lower the amount of circulating thyroid hormone- and vice versa. For those who do not have a functioning thyroid gland, the TSH responds to the amount of supplemented or exogenous thyroid hormone.
T4 - T4 is an inactive form of hormone produced by the thyroid gland, also known as thyroxine. It is made of iodine attached to tyrosine, an amino acid.
T3 - many organ tissues including the brain and liver can remove one of the iodine atoms from T4. This new compound is called T3, or triiodothyronine. This is an active version of thyroid hormone that communicates with different organs in the body to regulate how they process energy.
Both T3 and T4 can be measured in free and total forms. Total denotes the amount of hormone that’s circulating in the body, whether attached or unattached to a protein. Free hormone is the portion that’s unbound to protein and therefore biologically active.
Say you have someone with an absent thyroid gland - removed surgically due to enlargement of the thyroid. If that person didn’t take any thyroid hormone to replace what they no longer could manufacture on their own, then they would have a high TSH and a low T4. If they started taking the correct amount of levothyroxine, both values would normalize. However if they took too much levothyroxine, you could expect the TSH to be low and the T4 to be high.
Think of your hormones as a thermostat. If everything is working correctly, your brain should be maintaining hormone levels in a narrow range, quickly kicking in when production of a hormone is veering off-track. This is called a feedback loop.
This principle applies to the vast majority of people with thyroid problems. However, if someone had a problem with the pituitary gland, the TSH may not be a reliable or accurate value anymore.
And if someone has a faulty thyroid, the pituitary gland can beg all it wants - it’s not going to result in adequate thyroid hormone being generated.
Next time you get your lab work, try to draw correlations between the TSH and T3/T4.
Coming up soon - primers on thyroid antibodies and common myths in thyroid management. Stay tuned!
Image courtesy of: Chapter 31 - Disorders of the Hypothalamic–Pituitary–Thyroid Axis, Editor(s): George Fink, Donald W. Pfaff, Jon E. Levine, Handbook of Neuroendocrinology, Academic Press, 2012, Pages 685-706.