What Are Normal Thyroid Levels?

Learn what “normal” means, and what you should aim for
What Are Normal Thyroid Levels?

What is the difference between normal and optimal laboratory levels?


Laboratory values that are considered to fall “within normal limits” are different for each laboratory, which makes following a standard range difficult. Reference ranges are based on a group of otherwise healthy people and take into consideration factors including age, sex, general health, and medical history. Your laboratory result is then compared to this reference range to determine if you fall within this range and are deemed “normal” or not.


Due to this discrepancy in reference ranges between labs, how they calculate a reference range and disagreement on what the upper and lower limits of the range should be, it is recommended that biomarker levels are interpreted as a part of the patient’s larger health picture. This way patients are not being treated based on a number, but rather by taking into account their overall health.


What are normal thyroid function levels?


As discussed above, “normal” levels for laboratory tests can differ among labs, but there are general ranges that only vary slightly from lab-to-lab. When you get your results from your at-home thyroid test kit, you will see your TSH, free T3, free T4, and TPO antibody levels. 


For thyroid stimulating hormone (TSH), which is the hormone that signals synthesis and release of T3 and T4 from the thyroid gland, the normal range is approximately 0.45-4.50 uIU/mL. 


Free T3 and Free T4, which are the hormones produced and released by the thyroid gland, have ranges of 2.0-4.4 pg/mL and 0.82-1.77 ng/dL, respectively.


There are a couple of antibodies that, if present, can cause suspicion for an autoimmune disease of the thyroid as there should be no antibodies attacking our own cells. Thyroid peroxidase (TPO) is an enzyme present mainly in the thyroid that is turned on by TSH and helps with the production of T3 and T4. Anti-thyroid peroxidase antibodies attack TPO resulting in decreased TPO activity and decreased production of thyroid hormones. The reference range for anti-thyroid peroxidase antibodies is 0-34 IU/mL.


The second antibody that attacks the thyroid and can lead to an autoimmune disease is antithyroglobulin antibody. Thyroglobulin is a protein produced by the cells of the thyroid that also assists in the production of thyroid hormones. Antithyroglobulin antibodies are not specific to one disease of the thyroid and are even found in 1 in 10 normal individuals. This makes them of limited value in patients with Hashimoto’s thyroiditis, but it is still important to make note of their role. The approximate reference range for antithyroglobulin antibodies is 0-0.9 IU/mL.


What optimal level should I aim for? 


Since the optimal thyroid function level is the level at which you feel your best at, it varies for each person. You and your provider should strive to reach this optimal level, adjusting your medication appropriately until you are at a level that makes you feel your normal self again. Based on the reference ranges given above for the “normal” reference range, the following gives the optimal range that most thyroid patients report feeling their best at: TSH 0.5-2.5 uIU/mL, free T3 3.8-4.4 pg/mL, free T4 1.4-1.77 ng/dL, anti-TPO 0.0 IU/mL, and anti-TG 0.0 IU/mL.

Again, these values are not an end-all-be-all and not everyone’s optimal level will fall between these ranges, but this gives you an idea of the discrepancy between the calculations that the labs make based on their overall patient population when making a reference range and how patients with hypothyroidism are feeling clinically.


Does my optimal level change at different times of life?


Your optimal thyroid level changes as your body and lifestyle do. One of the most common causes of change in thyroid hormones is stress, and one of the most common prolonged causes of stress on the body during a woman’s life is pregnancy.


Women need about 45% more thyroid hormone during pregnancy to maintain their optimal TSH level. In a healthy individual, the thyroid can keep up by producing more hormone. However, in the hypothyroid patient, the thyroid likely will not be able to increase production and even with levothyroxine supplementation, TSH levels may rise out of the patient’s optimal range.


In order to avoid ever experiencing symptoms of hypothyroidism during pregnancy, a patient should have her TSH checked before becoming pregnant if possible, and again six weeks after conception. The TSH should continue to be checked every six weeks throughout the pregnancy. It is not uncommon for the dosage of levothyroxine to be changed multiple times throughout the pregnancy to keep up with the stress the pregnancy puts on the body.


Beyond long term changes in your body, there is evidence that thyroid hormone levels naturally fluctuate and follow diurnal rhythm. You should talk to your doctor about how to best time your testing both in frequency and time of day.


Work with your provider to reach your optimal levels


The first time most patients with hypothyroidism have ever had their thyroid levels checked, they were already abnormal. Therefore, many people don’t know at what level they felt well. So, you must work with your provider to find out where your optimal levels fall. It takes approximately six weeks for thyroid medication to build up in your system, so don’t be discouraged when you don’t see results overnight. Be patient, as it may take time to find your perfect dose.


Your provider wants you to feel your best just as much as you do. So, don’t be afraid to express to them your symptoms and how you are feeling. Your optimal thyroid function level is not out of reach and together you can treat this disease.

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