Last week I shared a little bit on how to recognize the signs and symptoms of low thyroid function and this week we will explore which lab tests you need to determine if you have an underactive thyroid. Most people don’t know that multiple markers must be considered to truly analyze thyroid function, but typically doctors only measure one or two of them.
In this article:
- Meet the HPT Axis
- What thyroid blood tests do you really need?
- A note about “normal” lab tests
- A Self-test for underactive thyroid
Meet the Hypothalamus Pituitary Thyroid (HPT) axis
In previous posts I introduced you to the HPA (hypothalamus, pituitary, adrenal) axis. Well there’s another ring of communication that happens between the hypothalamus, pituitary, and thyroid glands. This one is cleverly named the HPT axis. In a nutshell, when the hypothalamus detects low levels of thyroid hormones it will send a message to the pituitary via the hormone thyrotropin-releasing hormone (TRH). This signals the pituitary to produce another hormone called thyroid-stimulating hormone (TSH), which then tells the thyroid to produce more thyroid hormones as needed.
Typically when doctors test for thyroid function, they only measure TSH. When thyroid hormones in the blood are low, the body will signal to make more and TSH will be elevated. When thyroid hormones in the blood are high then the body is not directed to make more and TSH levels are lower.
Measuring TSH is a good start but it doesn’t paint the whole picture. If you remember back to my first post, there are two different thyroid hormones T3 and T4. The thyroid produces about 80% T4 and 20% T3, but T3 is the most active form. Measuring TSH alone won’t tell you the concentrations of T4 and T3 or how well your body is converting T4 to T3.
What thyroid blood tests do you really need?
I’d like to note that some of the blood tests below are primarily used by practitioners of functional medicine to assess the overall performance of the thyroid gland and the usability of thyroid hormones. They may not be necessary to diagnose true thyroid disease. If you are a relatively healthy person but suspect that some of your symptoms may be caused by an underactive thyroid then consider the following tests:
TSH
Thyroid-stimulating hormone as mentioned above is a measurement that may indicate the demand being placed on the thyroid to produce more hormones. If the measurement is high then it shows you that there is not enough thyroid hormone circulating and the thyroid is being asked to make more. If the TSH measurement is low, then it can indicate that too much thyroid hormone is circulating and there is little demand on the thyroid to produce more hormones.
Free T4
Free T4 measures how much T4 is unbound and able to be used by bodily tissue (and potentially converted to T3). The measure of Free T4 alongside TSH can be a better indicator of thyroid function. If free T4 is high then it can potentially indicate an overactive thyroid and if it is low in can potentially indicate an underactive thyroid.
Free T3
Recall that T3 is the active form of thyroid hormone and is often converted from T4 in other areas of the body. Free T3 essentially measures the availability of your active thyroid hormone. If Free T3 is low then it may indicate that you are not converting T4 into T3 effectively. If it is high then it can potentially indicate hyperthyroidism.
Reverse T3
T4 can also be converted to Reverse T3 which is another inactive thyroid hormone. When RT3 is high then it may indicate that most of the T4 is being converted to RT3 instead of T3 so the active form of thyroid hormone is not sufficient. This is often the case when you have low thyroid symptoms but TSH and T4 results are normal.
Thyroid antibodies
The most common cause of primary hypothyroidism is Hashimoto’s thyroiditis, an autoimmune disease that attacks the thyroid and inhibits its function. It is estimated that 14 million people (most of which are women) have Hashimotos in the U.S. alone. Yet, many doctors don’t even test for Hashimoto’s without a specific request from their patient.
Because Hashimoto’s is an autoimmune disease it involves the production of abnormal antibodies that can be detected in the blood. Early detection of antibodies can prevent severe damage to the thyroid that requires lifelong medication. Here’s what to test for:
Thyroid peroxidase antibodies (TPOab) – These antibodies attack the enzyme Thyroid peroxidase that plays an important role in producing thyroid hormones.
Thryoglobulin antibodies (Tgab) – These antibodies attack Thyroglobulin which is a protein produced in the thyroid and is the main precursor to thyroid hormones.
A note about “normal” levels
When labs determine their ranges, they are essentially testing a very large sampling of people and then taking averages of the results to come up with high, normal, or low ranges. Even when your results fall within “normal” ranges, you must pay attention to where you fall on the normal spectrum. If your results are closer to the higher or lower end of normal (depending on the test) your thyroid may not be functioning optimally and symptoms still occur. Many times people are written off and told to “just wait and see” if they fall anywhere within the normal range. Again this is a reason to seek out a functional medicine practitioner who will take these things into consideration.
Try this self-test for underactive thyroid
If you think you might be hypothyroid then there is a simple test you can do at home to confirm your suspicions. To be clear, this is not to diagnose or fully analyze the function of your thyroid, but it measures one of the first symptoms of hypothyroid: low body temperature.
First thing in the morning before you even get out of bed, take a mercury thermometer and place it in your armpit for 10 minutes. Keep a daily log of your temperature for several days. If you are still menstruating, you will have natural temperature fluctuations during your cycle. Pay close attention to your temperature after the second and third days of your period. A reading below 97.8 degrees may indicate hypothyroid.
Another possible indicator of hypothyroid is a mid-afternoon temperature that is significantly lower than 98.6 degrees.
Work with your doctor or seek out a practitioner who can request and accurately interpret all of your thyroid labs. You know your body best, and if you are being told that everything is normal but you aren’t convinced, keep looking for answers. You don’t have to settle for a life full of symptoms and discomfort.
Sources:
https://www.endocrineweb.com/conditions/hashimotos-thyroiditis/hashimotos-thyroiditis-facts-tips
https://www.thyroid.org/wp-content/uploads/patients/brochures/FunctionTests_brochure.pdf
https://www.thyroid.org/patient-thyroid-information/ct-for-patients/january-2019/vol-12-issue-1-p-11-12/
https://www.medicalnewstoday.com/articles/163729
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