Comprehensive Approach to Thyroid Testing

Testing the thyroid properly requires more than just checking the level of TSH and free T4. Those are the most often checked by doctors to estimate if the thyroid functions properly, but it does not show the true picture of the thyroid function.

Here, you can find the complete list of thyroid labs and a short description to help you understand the roles and functions of each hormone from the list below.

  • TSH -Thyroid stimulating hormone
  • Free T3 (Triiodothyronine, free)
  • Free T4 (Thyroxine, free)
  • Reverse T3 (Triiodothyronine, reverse)
  • Total T3 (Total triiodothyronine)
  • Thyroglobulin Antibody( TgAb)
  • Thyroid Peroxidase Antibody (TPO)

The thyroid gland hormone production is regulated by the hypothalamus which is located in your brain and the pituitary gland. Hypothalamus releases thyrotropin-releasing hormone, which then causes the pituitary gland to produce the Thyroid Stimulating Hormone (TSH).

TSH changes its levels in response to the need of the amount of hormone, T4, to be produced by the thyroid. If the thyroid does not produce enough T4, TSH will raise to let the thyroid know that it is time to produce more. TSH will drop in response to the higher amount of this thyroid hormone in the system so production of T4 can slow down.

The thyroid gland mainly produces the hormone T4. T3 is also produced by the thyroid, but in very low amounts as T4 can be converted into the active T3 in other tissues. The thyroid requires iodine in order to produce the main thyroid hormone, T4.

T4 is an inactive thyroid hormone which is converted, mainly in the liver, into the active thyroid hormone, T3. In order for this process to occur, our body requires an enzyme that depends on selenium. If the conversion of T4 to T3 is poor, then you may experience some of the common hypothyroid symptoms like depression, sensitivity to cold temperatures, cold feet and hands, difficulty concentrating, fogginess in your head, forgetfulness and fatigue.

Free T3 (FT3) is the active and circulating form of thyroid hormones. Low T3 indicates your body is not converting enough T4 into Free T3. This can be due to stress, impaired liver function, problems with the GI system, restricted calorie diet or selenium deficiencies. It is a very common cause of hypothyroidism.

Reverse T3 (RT3) is the inactive T3 metabolite. RT3 is considered an unusable form. If more RT3 attach to cell receptors than Free T3, it slows down your metabolism. We want to have more active T3 than Reverse T3. Stress can disturb that delicate balance between the active and reverse T3 form. During stress, our body releases cortisol, the stress hormone. In stressful periods, our body slows down metabolism to have energy for other more important processes. These responses are adaptations to increase our chances of survival in threatening situations (i.e. the fight or flight response). Higher Reverse T3 can also be found in the period of starvation and very restrictive diets.

A high level of thyroid antibodies, TPO and/or Thyroglobulin, may indicate an ongoing autoimmune attack. It can cause autoimmune hypothyroidism, a condition known as Hashimoto’s disease. In Hashimoto’s disease, the thyroid does not produce enough thyroid hormone or does not convert t4 into t3 properly. Hyperthyroidism, on the other hand, is when your thyroid overproduces thyroid hormone. It is called Graves’ disease.

It is important to check your thyroid antibodies even if your TSH has been in normal ranges. It may take years for the body to start showing lower hormones, T3 and T4, and elevated TSH, in the case of Hashimoto’s disease.

Functional Medicine adapted narrower reference ranges for all of the above labs. It is important since many doctors know that those diseases could be diagnosed much earlier if we look at narrower ranges.

Lab Test


Normal Ranges

Optimal Ranges

Thyroid Stimulating Hormone


0.45 – 4.5 uIU/ml

1 – 2.0 uIU/ml

Free T3 Triiodothyronine


2.0 – 4.4 pg/mL


Free T4, Thyroxine


0.82 – 1.77 ng/dL

>1.1 ng/dL

Total T3, Total Triiodothyronine


71 – 180 ng/dL

150 – 180 ng/dL

Reverse T3, Reverse Triiodothyronine


9.2 – 24.1 ng/dL

<10:1 ratio RT3:FT3

Thyroglobulin Antibodies


0.0 – 0.9 IU/mL

<0.9 IU/ml

Thyroid Peroxidase Antibody


0.0 – 0.9 IU/mL

<4 IU/ml

From an integrative medicine point of view, when assessing the thyroid, I often recommend checking nutritional status as nutrients play a very important role in thyroid function.

I hope this article will help you understand the importance of a comprehensive approach to your thyroid function. If your doctor does not want to order all the labs you feel are needed, you can order an affordable test without a doctor’s order using the button below (Labs are available for U.S. residents only, except NY, NJ, and RI residents).


The information provided on this website and my services are not intended to replace a one-on-one relationship with a qualified Medical Doctor and is not intended as medical advice. It is intended as sharing of knowledge and information from my own research, experience and other experts in this field. I encourage you to make your own health decisions based on your own research and in partnership with a qualified healthcare professional. The articles on this website are provided for informational purposes only and no promises, guarantees, representations or warranties of benefits have been made. I shall have no liability for any failure or success and the use of this information.


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