When I treat patients with thyroid disorders, one of the very first things we do is run a full assessment of their thyroid function, looking at various blood markers such as TSH, T4, T3 and thyroid antibodies. These blood tests give us a complete picture of how well the thyroid is functioning and what might be going on under the surface. From there, we can structure an individualised treatment approach and use follow-up blood tests to monitor progress.
In other cases, patients come to me with a set of symptoms highly suggestive of thyroid dysfunction, and yet they’ve been told their thyroid function is ‘normal’. Often these patients have only had one marker of thyroid function tested (usually TSH), but other important indicators like T4, T3 and reverse T3 have not been assessed. It’s an incomplete picture and sadly, the patient is often still struggling with weight management, low energy and fatigue as a result.
One of the first things I do with thyroid patients in clinic is teach them what the different results on their thyroid function tests mean, so they can be more involved and empowered in their health journey. In this article today, I want to do the same for my readers, as being able to understand your results enables better communication with healthcare providers and a greater understanding of your health in general.
Note: In this article, I won’t be using specific number values, as the reference ranges provided on blood tests vary greatly depending on the method of analysis, the lab used and the units of measurement.
Thyroid Stimulating Hormone (TSH)
TSH, or thyroid stimulating hormone, is the most common blood test used to assess thyroid function. It’s often used to screen for thyroid dysfunction.
TSH is produced in the brain to stimulate the thyroid to produce thyroid hormones. TSH production depends largely on how much active thyroid hormone is in the blood. If we have too little thyroid hormone in the blood, the brain produces even more TSH, to further stimulate the thyroid to produce more thyroid hormone. If we have too much thyroid hormone in the blood, TSH production declines, so the thyroid receives less stimulation from the brain and thus produces less thyroid hormone.
It’s a little bit like a jockey cracking a whip, where the brain is the jockey, TSH is the whip and the thyroid is the horse: If the horse is going too slowly (e.g. the thyroid is under-active), the jockey will crack the whip harder and faster (TSH production will increase).
It’s important to have enough TSH, to prevent atrophy (wasting away) of the thyroid. On the other hand, too much TSH can cause the thyroid to grow and even form nodules, which can also be a problem. Ultimately, we want enough TSH to ensure adequate stimulation of the thyroid to allow for healthy thyroid hormone levels.
Free T4 (T4)
T4 (or thyroxine) is the main hormone produced by the follicular cells of the thyroid, in response to TSH. T4 consists of four iodine molecules, bound by the amino acid tyrosine. It is released into the blood, where it is converted in cells to T3 (see below).
As T4 is predominantly made up of iodine, a deficiency of this nutrient can lead to reduced T4 production. Other factors that can also reduce T4 production include autoimmune thyroid disease (e.g. Hashimoto’s), heavy metal exposure and chronic disease. For this reason, if T4 levels are low, it is important to identify why. Depending on the situation, patients with low T4 production are often prescribed levothyroxine sodium, which is a synthetic, albeit bio-identical form of thyroxine. This is referred to as ‘thyroid hormone replacement therapy’.
Free T3 (T3)
After T4 is released by the thyroid, it is carried in the blood by carrier proteins such as thyroxine-binding globulin, transthyrein and albumin. It is then released from these carriers into cells (such as in the liver and kidneys), where it is converted into free T3 (or triiodothyronine). T3 is three-to-four times more active than T4, so it is considered the ‘active form’ of thyroid hormone.
Conversion of T4 to T3 requires key nutrients like selenium and zinc, so when we assess thyroid function we typically conduct a nutritional assessment as well, to ensure adequate nutrients are available for conversion. Key nutrients to look at in terms of thyroid function include iodine, iron, zinc, vitamin D and selenium.
As I mentioned above, the brain ‘reads’ how much thyroid hormone is in the blood, and adjusts TSH production accordingly. Essentially, TSH represents how the brain is communicating with the thyroid; T4 tells us how much thyroid hormone is being produced, and T3 tells us how well that thyroid hormone is being converted into its active form. This is why we often order a full thyroid function assessment, so we can understand the complete picture and identify where in the process dysfunction may be occurring and why.
Reverse T3 (rT3)
When T4 is converted to T3, a portion is also converted to a hormone called reverse T3. While T3 is the active form of our thyroid hormone, reverse T3 is considered biologically inactive. During times of stress, T4 ‘shunts’ towards reverse T3, meaning reverse T3 levels increase and there is less active thyroid hormone available. In addition to times of stress, other conditions that can increase reverse T3 production include inflammation, starvation, fasting and/or chronic disease (especially liver and/or kidney disease).
Sometimes T3 levels can be lower than expected, despite sufficient levels of T4, selenium and zinc. In this case, T4 may be shunting towards reverse T3, so blood tests and other clinical investigations may help determine why.
Sometimes, the immune system may produce cells which attack thyroid tissue, causing damage and inflammation. These ‘deranged’ immune cells are called thyroid antibodies, and they are associated with various autoimmune diseases, such as Hashimoto’s disease and Graves’ disease. There are various types of thyroid antibodies, such as thyroid peroxidase antibodies, thyroglobulin antibodies and thyroid receptor antibodies. Each of these affects the thyroid and thyroid hormone production in different ways, so it is important to speak to your healthcare professional if you have raised levels of any of these thyroid antibodies, so they can recommend appropriate monitoring and treatment.
Thyroglobulin is a protein produced by the follicular cells in the thyroid, to store thyroid hormones before they are released into the blood. When needed, thyroglobulin is broken down, allowing for the release of T4 (and some T3) into the bloodstream. Thyroglobulin is not commonly tested as part of a thyroid function assessment, but may be evaluated in patients who have undergone treatment for thyroid cancer, to evaluate the efficacy of treatment.
The Take Home Message
Overall, there are many tests that can be used to identify whether thyroid dysfunction is occurring and why. TSH is the most commonly tested marker of thyroid function, but represents only one piece of the puzzle. Being able to understand and interpret your thyroid function results allows you to communicate more effectively with your healthcare provider and be more informed and proactive in your healthcare. Hopefully this article has been helpful in clarifying what some of the different thyroid tests mean, and why they might be ordered. As always, if you are concerned about your thyroid health or function, I recommend speaking to a qualified healthcare professional. Stay tuned for more articles on thyroid health and function, or check out my Blog section to read some of my existing posts on the topic.
Yours in health,