Skeleton with thyroid gland superimposed and labelled with T3, T4, TSH, rT3, and TP0Ab Anti-TG and text bubble reading Basic Thyroid Lab Testing

Lab testing is commonly performed to assess and diagnose thyroid disease. We use these tests to look for hyperthyroidism, hypothyroidism and other thyroid related conditions.

Commonly Run Thyroid Function Tests

Thyroid function tests are the blood tests done to assess thyroid function; they are one of the most requested lab investigations.

In the majority of cases, the results of thyroid function tests (TFTs) are straightforward, presenting a pattern that is readily recognized and consistent with overall thyroid health [2].

Below are the most commonly run thyroid function tests: TSH, T3, T4 and reverse T3.

Thyroid Stimulating Hormone (TSH)

TSH is routinely run to assess thyroid health.  Even minor changes in Thyroid hormone (T3 and T4) will change TSH levels significantly [2]. Some doctors and labs will offer T4 and T3 for a more comprehensive look, but in most cases, TSH is measured alone.

TSH Regulation

If Thyroid hormones (T3 & T4) are low a signal is sent to the pituitary to make more TSH. TSH will then signal the thyroid gland to make more T3 and T4. This is what happens in a hypothyroid state.

Likewise, if thyroid hormones (T3 & T4) are high a signal is sent to the pituitary to reduce TSH. This means the thyroid gland will stop making T3 and T4. This is what happens in a hyperthyroid state.

A lot of doctors will rely solely on TSH to assess thyroid function. If TSH is high, the patient has low functioning thyroid; if TSH is low, the patient has an overfunctioning thyroid gland. Measuring only TSH works most of the time, but there are good reasons to not solely rely on TSH to assess thyroid function.  Click here for an article outing the pitfalls of relying solely on TSH.

a schematic showing thyroid metabolism in hypothyroidism, including T3 and T4 and how when they drop, TSH will increase
thyroid physiology involved in hyperthyroidism showing T3 and T4 hormones have increased expression while TSH drops

T3 and T4 (Free and Total)

As mentioned above, thyroid hormones exist as T3 and T4. Furthermore, these hormones exist in two forms: free and total. Some of the hormones in the blood are floating around free and unattached, while others are floating around bound to carrier proteins.

When we test thyroid hormones, it’s important to distinguish T3 from T4, and to know if they are free or bound.

Total T4 and total T3 measurements are less accurate because several medications and medical conditions can interfere with the protein carriers that bind to thyroid hormones and confuse results [3].

Free hormone levels are preferred as they are more indicative of true thyroid function. Measurement of the total levels is still frequently used because many laboratories cannot accurately measure free thyroid hormone levels [3].

The images below depict the concept of thyroid hormone carrier proteins.


thyroid gland physiology showing the effects of bound hormones as it relates to the results of basic thyroid lab testing

Click here to learn more about T3 and T4 and here to learn more about protein carriers which affect these hormones.

Reverse T3 (rT3)

Click here to review the concept of thyroid hormone conversion. Reverse T3 is another product that can be made from T4. Similarly, to how T3 is made through conversion of T4 through enzymatic reactions, reverse T3 can also be made.

The difference between T3 and rT3 is that rT3 is not biologically active, yet it can bind where T3 normally does. It is a defense mechanism, when there is enough T3 in the system, rT3 will be made to protect the body from over stimulation of thyroid hormone.

Unfortunately, there can be faulty rT3 production too. This is often the case in a situation called Low T3 Syndrome – where there is adequate TSH and T4, but not enough T3 in the system. Various factors, such as stress, inflammation and certain nutrient deficiencies, can cause pathological amounts of reverse T3 to be created.


Antibodies are often tested (in the blood) when autoimmune disease is suspected. There are different antibodies that can be tested, and the presence of each one points to a different pathology or disease.

Summary of Normal Findings of Common Tests

TSH – Thyroid Stimulating Hormone Free T4 – Thyroxine Free T3 – Triiodothyroxine Reverse T3- rT3 Antibodies
· Released by the Pituitary gland

· Stimulates the thyroid gland to make Thyroid hormones (T3 & T4)

·  If TSH is high, it implies that T3 and T4 must be low (hypothyroidism)

· IF TSH is low, it implies T3 and T4 must be too high (hyperthyroidism)

·  One of two thyroid hormones produced by the thyroid gland

·  T4 is made in higher proportions than T3 but T3 is more active. 85-90% of the hormone the thyroid gland produces is T4.

·  Once it gets to the target cells, T4 must convert to T3

·      One of two thyroid hormones produced by the thyroid gland

·      The other thyroid hormone, T4, will be converted into T3 (enzyme dependant).

·      This is the one that is more active, and imparts the functions of the thyroid on the target organs

·       When there’s sufficient T3, the body will convert excess T4 into rT3.

·       rT3 will bind to the same receptors of T3 but has no action

·       rT3 can be made in normal situation or can be made in excess due to other reasons

Thyroid Peroxidase Ab

·    These target the enzyme (thyroid peroxidase) which converts T4 into active T3


Antithyroglobulin Ab

·    This targets a protein called Thyroglobulin, which is present in the thyroid gland.

·    The auto-immune destruction will cause damage to the thyroid gland itself, resulting in altered thyroid hormone output.

Normal Range:

0.4-3 mlU/L


“Naturopathic”  Range: Below 2.5 mlU/L


Fertility Range: <2 mlU/L

Normal Range: 10-23 pmol/L


“Naturpathic Range”

18-23 pmol/L

Normal Range: 3.5-6.5 pmol/L Normal Range: 200-300 pmol/L Normal Ranges

TPO Ab: <35

Anti-TG: <20

Optimal T4/T3 range is about 5:1 Women who test positive for these anti-bodies will generally develop hypothyroidism at a rate of 20% per year. Not everyone with elevated Ab will even show signs of hypothyroidism, It’s important to know for when a woman wants to become pregnant.

*Naturopathic Ranges: these are values that I often use instead of the lab reported ‘normal values’. It’s important to realize that the values of reference ranges are based on being ‘disease free’. Many people believe there is a significant difference between being ‘optimal’ and ‘disease free’, hence the different ranges for TSH and T4.

When Labs Don’t Make Sense

As mentioned above, most cases are straightforward. Typically, the lab values match with symptoms and the lab values correlate to each other.

A small but significant number of cases don't follow the rules of normal blood work [4]. Sometimes patients have abnormal TSH and normal thyroid hormones, or abnormal thyroid hormones yet normal TSH. These results are often regarded as confusing, but many patients are treated using conventional approaches anyway; it is important for these cases to be investigated further to ensure the treatment prescribed is appropriate for you.


Interested in learning more?

Read on in our series of articles on Thyroid Health!

About the Author - Dr. Johann de Chickera

man facing camera

Dr. Johann is a fully licensed Naturopathic Doctor. His approach emphasizes the importance of living a healthy lifestyle and improving one’s health naturally. Dr. Johann obtained a Doctor of Naturopathy at the Canadian College of Naturopathic Medicine (CCNM). Education at CCNM is a vigorous four years, with a curriculum involving biomedical sciences, physical diagnosis, clinical nutrition, traditional Chinese medicine and acupuncture, botanical (herbal) medicine, physical medicine, homeopathy and lifestyle management.

While Dr. Johann has a general practice, he focuses on fertility, hormonal imbalances, gut health, and autoimmune disease.

To book in please call us at (519) 442-2206 or click here.

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