Decoding thyroid testing

Lab testingDo you feel like thyroid lab testing is a foreign language?

You’re not alone!

One of the most confusing aspects of thyroid dysfunction is understanding thyroid testing. What tests have been done and what do they mean? What other testing can be helpful to gain a deeper understanding of thyroid function? This blog is about decoding thyroid testing, teaching you the basics of thyroid function and explain the meaning and purpose behind both common and less common testing available.

All about your thyroid

The thyroid is a butterfly-shaped gland found at the base of the throat that plays a major role in regulating temperature, basal metabolic rate, and nearly every other tissue and system in the body! It can affect digestion, the nervous system, heart health, musculoskeletal system, reproduction and fertility, and skin.

You’ve probably heard the terms “hypothyroidism” and “hyperthyroidism” before, right? Since they sound nearly EXACTLY the same, sometimes people get confused, so here is a comparison:


Hypo means under, or less than normal, so hypothyroidism is less than normal functioning of the thyroid gland. Symptoms of hypothyroidism are the result of this slowed down or sluggish function and include: fatigue, cold intolerance, cold extremities, low libido, infertility, thinning eyebrows, dry skin, hair loss, brittle nails, poor memory, constipation, heavy periods, irregular periods, menstrual cramps, weight gain (but can see weight loss), puffy eyes, elevated cholesterol.


Hyper means the opposite of hypo: above or higher than normal, so hyperthyroidism is excessive or higher than normal functioning of the thyroid gland. Symptoms of hyperthyroidism are therefore the results of this speeded up function and include: fatigue, sweating, heat intolerance, flushing, nervousness, anxiety, heart palpitations, increased heart rate and blood pressure, oily skin, insomnia, tremor, irregular periods, weight loss (despite normal or high appetite), goiter, protruding eyes.

The hormone cascade:

Your brain (specifically the hypothalamus) make a hormone called thyrotropin releasing hormone (TRH) that stimulates another part of your brain (this time the pituitary gland and specifically the anterior part of it) to make thyroid stimulating hormone (TSH). TSH causes your thyroid gland to make thyroid hormones which have action in your body tissues.

Thyroid hormones are called thyroxine (T4) and triiodothyronine (T3). T4 (which is a prehormone) is released in greater quantities by the thyroid because it is converted into T3 by tissues. T3 is much more active than T4 and most of it is produced through this peripheral conversion of T4. These thyroid hormone levels also provide a feedback signal to both your hypothalamus and pituitary gland as to whether they need to increase or decrease their production of their hormones respectively.

Decoding thyroid testing

Thyroid stimulating hormone (TSH):

The most basic screening test for thyroid function is TSH, which is measuring production by the pituitary gland as it aims to stimulate the thyroid gland. Therefore, a HIGH TSH level indicates an UNDER-functioning thyroid (hypothyroidism); the high TSH level means that the pituitary gland is over-producing TSH trying to stimulate a thyroid gland that may not be responding adequately. The reverse is true for hyperthyroidism: TSH will be LOW, indicating an OVER-functioning thyroid gland. Just remember that a high number means you’re low and vice versa!

Free thyroxine (free T4):

This is the next most common test you will likely see as this is the thyroid hormone produced in larger quantities by the thyroid gland. T4 is converted to the more active triiodothyronine (T3) in your body tissues. “Free” means this test measures the metabolically active hormone; a total T4 test measures both free hormone and hormone bound to a carrier protein (thyroid binding globulin).

Free triiodothyronine (free T3):

This test less common, but is useful for measuring the second hormone produced by the thyroid gland and actual action of thyroid hormones in your body. “Free” means this test measures the metabolically active hormone; a total T3 test measures both free hormone and hormone bound to a carrier protein (thyroid binding globulin).


You’ve probably heard of autoimmune disease, right? Just to refresh, autoimmune disease occurs when your body makes antibodies (part of your body’s immune response) to tissues or substances that are part of you. This means your body is attacking itself and causing damage.

Thyroid dysfunction can be caused by an autoimmune process, so it is important to test antibody levels to determine whether this is the case because it helps us understand more about why a patient has thyroid dysfunction and guides treatment decisions. Autoimmune hypothyroidism (called Hashimoto’s) is the most common form of hypothyroidism. Graves disease is the name for autoimmune hyperthyroidism.

There are many types of antibody tests, but below I have given you the basics on the two most common.

Antimicrosomal/Antithyroid peroxidase:

These antibodies are found in most patients with autoimmune thyroid dysfunction, usually Hashimoto’s hypothyroidism.

Antithyroid peroxidase antibodies (anti-TPO) is the most sensitive test for antimicrosomal antibodies. Anti-TPO is present in nearly all Hashimoto’s patients and is also present in a high percentage of Graves patients. It can also be present in patients with other autoimmune diseases. This test provides a measure of the degree of inflammation/attack occurring in the thyroid.

Antithyroglobulin antibody:

This is another common thyroid antibody test that is found less often in autoimmune thyroid conditions. However, it is a good idea to test both antithyroid peroxidase (anti-TPO) and antithyroglobulin in conjunction to maximize sensitivity and specificity and correctly and accurately diagnose thyroid dysfunction.

Other useful tests:


Taking an ultrasound of the thyroid gland helps visualize abnormalities of the thyroid, such as nodules or cancer as well as measure the extent of any thyroid enlargement/inflammation.

Reverse T3 (rT3):

Reverse T3 is a virtually inactive form of T3 that binds to T3 receptors and therefore blocks the action of normal T3. If this type of T3 is too high, other thyroid testing does not give a true picture of thyroid function. This is much less commonly used test but can be very useful when you want to delve deeper to understand thyroid function and to the best treatment for you.

Thyroid binding globulin:

This tests the amount of the protein that carries thyroid hormones to tissues and helps to understand whether an abnormally high amount of thyroid hormone is bound up and therefore unavailable. When this happens, TSH will increase to produce more T4 and T3 to increase the amount of hormone that is free, or metabolically active.

Gluten sensitivity testing:

As discussed in my blogs on gluten sensitivity, thyroid dysfunction (autoimmune and otherwise) is correlated with gluten sensitivity and in some cases, identifying this sensitivity can aid in regulating thyroid function. For more on this subject, see my blog on gluten sensitivity diagnosis and why gluten sensitivity matters.

Watch my video about how Naturopathic medicine can treat Hashimoto’s Hypothyroidism:


  1. Are You Prescribing the Wrong Thyroid Medication? - Naturopathic Doctor Development Center - November 10, 2015

    […] This article is about the surprisingly few medications available to treat underactive thyroid (aka hypothyroidism) as well as how to optimize treatment whether you are taking thyroid medication or not.  Learn more background information on decoding thyroid testing. […]

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