Why TSH is not enough for thyroid disease diagnosis.
A common thing I hear from patients presenting with signs of thyroid disorders is that their doctor already checked their thyroid levels. I usually follow up by asking what exactly was tested and their response is “just TSH (thyroid stimulating hormone)”. I have a hard time understanding why it is not more common to check all thyroid hormones as TSH is a reflection of what’s going on with the pituitary gland, which is located at the base of the brain, not the thyroid itself. It is also fairly inexpensive and most insurance covers the cost of these lab markers. I’m a visual person so I show my patients this simple diagram below to explain why we look at more than just TSH in our office.
TSH is released from the pituitary gland (located at the base of the brain) and acts on the thyroid gland itself. The thyroid then releases thyroxine (T4), the inactive thyroid hormone. With T4 it can be further analyzed into Total T4 and Free T4. The difference between Total T4 and Free T4 is that the total looks at the amount of T4 bound to cells and free T4 is what is still available to be used by the body. T4 is then converted to triiodothyronine (T3) to be utilized by the cells. T3 can also be measured in total and free T3. Reverse T3 (rT3) comes from T4 as well. The issue is that rT3 can block the binding sites on cells, inhibiting T3 from binding. So your T3 levels may be acceptable based on blood work but they may not be able to bind due to rT3 giving you signs of hypothyroid (slow functioning thyroid). Now rT3 is normal at a certain level, but it becomes an issue when it's too high. I also check for thyroid antibodies (Anti-TG and TPO antibodies) as these are usually the culprit in Hashimoto’s and Grave’s disease (which are thyroid diseases). These antibodies mistakenly attack the thyroid tissue resulting in an autoimmune disorder.
That last paragraph may be a bit difficult to digest but ultimately you can see that just looking at just TSH does not give you the whole picture of what is going on in regards to the thyroid. TSH could be completely normal and T4 or T3 could be out of range, but we wouldn't know unless we looked at the whole thyroid panel. If I haven't lost you yet, I've got more. Different levels of TSH, T4, and T3 can mean a variety of things. When the thyroid makes too little hormone it is classically called hypothyroid, when it makes too much it's called hyperthyroid. Both of these various dysfunctions result in very distinctive signs and symptoms.
A variety of things affect the thyroid and thyroid hormones, which goes back to my point in determining the exact point in this chain of events that’s the issue. Nutrients including selenium and zinc are important in converting T4 to T3. The thyroid can benefit from iodine, some B vitamins, vitamin C and D to produce T4. Things that encourage conversion of T4 to rT3 (blocking the cell from receiving T3) are things including stress, low calorie diets, inflammation, some medications and the list goes on. To increase cells sensitivity to T3 are exercise, vitamin A and zinc.
Have I convinced you yet on why TSH alone is not enough for diagnosing and medicating someone with thyroid dysfunction? I will leave you with the list of markers to ask your doctor to run. I send most of my patients back to their primary care doctors since most insurance will cover these (I do not accept insurance for functional medicine in my office) and I've had no issues with PCP’s running them. I hope this blog empowers you, if you suspect you may have some sort of thyroid dysfunction to be your own advocate and ask your doctor for the FULL panel!
A full thyroid panel: TSH, Total T4, Free T4, T3, Free T3, Reverse T3, Anti-TG and TPO antibodies.
*Most of this information came from my UWS Master’s program and information from the Institute for Functional Medicine*
Armstrong M, Asuka E, Fingeret A. Physiology, Thyroid Function. [Updated 2021 Mar 23]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2021 Jan-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK537039/
Shayri M. Kansagra, BS, Christopher R. McCudden, PhD, Monte S. Willis, MD, PhD, The Challenges and Complexities of Thyroid Hormone Replacement, Laboratory Medicine, Volume 41, Issue 6, June 2010, Pages 338–348, https://doi.org/10.1309/LMB39TH2FZGNDGIM