Understanding what type of thyroid condition seems to be a common question I receive. One of the questions I am most frequently asked is, “what are the most important thyroid function tests to check to assess your thyroid’s function?” The appropriate tests, along with what your thyroid lab results really mean, are two of the essential topics to understand…to determine if you genuinely have thyroid dysfunction or not. So I thought I’d put together this reference guide to help. With this table in mind, let’s discuss what each lab test means.

TSH

TSH tests pituitary function and can be used to diagnose thyroid disease.

• If TSH is high – this can be a sign that you are under-producing thyroid hormones and you are hypothyroid
• If TSH is low – this can be a sign that you are over-producing thyroid hormones and are hyperthyroid, or that you are on too much supplemental thyroid hormone. Supplemental T3 or natural desiccated thyroid hormone with T3 can artificially suppress your TSH, so in the absence of symptoms, it could be perfectly normal.
• If your TSH is ‘normal’ – for example your TSH falls within the normal reference range, this could indicate that you do not have thyroid dysfunction. However, “normal” and “optimal” levels have two very different meanings. So, if you still have symptoms and are in the “normal” — not “optimal” — range then you likely could have thyroid dysfunction

Free T3

Free T3 may be the most important measure of thyroid function in the serum because it measures the free and active thyroid hormone. T3 is more biologically active than T4.

• If FT3 is high – indicates that your thyroid is overactive or hyperthyroidism
• If FT3 is low – you may not be converting T4 to FT3 very well and you could have hypothyroid symptoms even if your TSH and FT4 are within range. This is one of the most common causes of low thyroid or hypothyroidism I see in my practice.

Free T4

Free T4 measures the amount of free T4 in circulation. Once TSH signals to your thyroid to ramp up the production of its hormones, it produces the four different types of thyroid hormone – T1, T2, T3, and T4. The primary output of your thyroid is T4, which is a storage form of the hormone. It is circulated throughout the bloodstream and stored in tissues so that it’s available when needed. I liked to measure Free T4 (FT4) since it is unbound and able to act in the body.

• If FT4 is high – it can indicate an overactive thyroid or hyperthyroidism
• If FT4 is low – it can indicate an underactive thyroid or hypothyroidism

Total T3

Total T3 includes measurement of bound T3. Bound T3 is not considered active like free T3 but total T3 gives you a more stable long-term marker of T3 in circulation.

Thyroid Antibodies

The thyroid antibodies include thyroglobulin antibody, thyroid peroxidase antibody and thyroid stimulating immunoglobulin. The presence of these antibodies in your serum may indicate an autoimmune disease which is damaging your thyroid gland. Thyroid peroxidase antibodies (TPOAb) attack an enzyme used to synthesize thyroid hormones and are commonly elevated in both Hashimoto’s and Graves’ Disease patients. Thyroglobulin Antibodies (TgAb), attack thyroglobulin, which your thyroid uses to produce its hormones. These are typically elevated in Hashimoto’s patients.

• If your antibodies are elevated – your immune system is attacking your thyroid and you have autoimmune thyroid disease, or you are on the autoimmune spectrum.

Reverse T3

Reverse T3 helps measure the conversion capacity of your thyroid gland. Your body also uses a portion of the T4 to create Reverse T3 (RT3), another inactive form of thyroid hormone. RT3 can attach to the receptors for Free T3 to slow down your metabolic processes.

• If RT3 is high – you are likely converting too much T4 to RT3 and not enough to FT3, which can cause hypothyroid symptoms even if your TSH and T4 levels are optimal. In addition, I look at something called the RT3/FT3 ratio. I like that to be less than a 10:1 ratio.

What you Should have Checked

Most conventional medicine doctors only check your Thyroid Stimulating Hormone (TSH) levels. If you are lucky, they will test your Free T4 levels to see if you are low on the storage form of thyroid hormones. However, as we’ve just covered, there are many factors involved in optimal thyroid function, so those two levels alone don’t tell the whole story. To get a complete picture of a patient’s thyroid health and medication needs, I recommend ordering all of the thyroid tests listed below:

• TSH
• Free T4
• Free T3
• Reverse T3
• Thyroid Peroxidase Antibodies (TPOAb)
• Thyroglobulin Antibodies (TgAb)

Optimal Thyroid Lab Ranges

In my Practice, I have found that the ranges below are the ones in which my patients thrive. I listen to my patients as well and take how they are feeling into account.

• TSH 1-2 UIU/ML or lower (Armour or compounded T3 can artificially suppress TSH)
• FT4 >1.1 NG/DL
• FT3 > 3.2 PG/ML
• RT3 less than a 10:1 ratio RT3:FT3
• TPO – TgAb – < 4 IU/ML or negative

Being educated about what’s going on with your thyroid will help you make the best choices when determing the next step in taggling your thyroid questions!