Thyroid Function Test Levels and Lab Values

Understanding Thyroid Blood Tests
It's important to know how conventional and integrative physicians interpret your thyroid tests.. David Silverman Getty Images News

If you want to understand key thyroid function tests and lab values, and what they mean, here is a summary of the key tests, what they measure, the reference range, and how they are interpreted by conventional and integrative physicians.

Note: The reference ranges and units of measurement used can vary from lab to lab. Always check to find the specific reference ranges and test values at the laboratory where your tests are conducted. 

TSH -- Thyroid Stimulating Hormone / Serum Thyrotropin

Measures: A pituitary hormone that responds to levels of circulating thyroid hormone.
Reference Range: 0.5 - 4.70 µIU/mL. (Some laboratories are .3 to 4.5, or other similar ranges.)
Conventional Interpretation: Above range, and under 10 µIU/mL is "subclinical" hypothyroidism, over 10 µIU/mL is overt hypothyroidism. Under .1 to .5 µIU/mL is considered evidence of subclinical hyperthyroidism, less than .1 may be overt hyperthyroidism. "Normal" TSH levels are considered to exclude hypothyroidism or hyperthyroidism.
Integrative Interpretation: Levels above 1.5 to 2.0 µIU/mL may be indicative of thyroid dysfunction. Optimal level is from 1.0 to 1.5 µIU/mL.
 

T4 - Thyroxine / Free Thyroxine - Free T4

Measures: Total T4 measures the total amount of thyroxine circulating in the bloodstream. Free T4 measures the available, unbound amount of thyroxine in the bloodstream.

A healthy thyroid gland produces mainly thyroxine, and the thyroxine needs to be converted into triiodothyronine (T3) in order to deliver oxygen and energy to cells.
Reference Range: Total T4: 4.5 - 12.5 µg/dL, Free T4: 0.8 - 1.8 ng/dL
Conventional Interpretation: Many conventional physicians do not test Total T4 or Free T4.

However, in some cases, along with elevated TSH, Total T4 or Free T4 levels that are below the reference range are considered evidence of hypothyroidism. Along with low/suppressed levels of TSH, Total T4 or Free T4 levels that are above the reference range are considered evidence of hyperthyroidism.
Integrative Interpretation: For diagnosis and treatment of hypothyroidism, levels in the top half of the reference range are considered optimal and evidence of proper thyroid function.
 

T3/FT3 - Triiodothyronine / Free Triiodothyronine

Measures: Total T3 measures the total amount of triiodothyronine circulating in the bloodstream. A healthy thyroid gland produces some triiodothyronine -- the active thyroid hormone. The rest is the result of conversion of thyroxine into triiodothyronine.
Reference Range: Free T3: (Triiodothyronine): 2.3- 4.2 pg/mL, Total T3: 80 -200 ng/dL
Conventional Interpretation: Many conventional physicians do not test Total T3 or Free T3. However, in some cases, along with elevated TSH, Total T3 or Free T3 levels below the reference range are considered evidence of hypothyroidism.

Along with low/suppressed levels of TSH, Total T3 or Free T3 levels that are above the reference range are considered evidence of hyperthyroidism.
Integrative Interpretation: For diagnosis and treatment of hypothyroidism, levels in the top half of the reference range are considered evidence of sufficient thyroid function, and levels in the top 25th percentile of the reference range are considered optimal. In the integrative view, Sub-optimal levels may warrant treatment with thyroid hormone replacement medications, or a medication that includes T3 specifically.
 

RT3 - Reverse Triiodothyronine - Reverse T3

Measures: An inactive, useless form of T3 that is produced when the body is under stress.
Reference Range: Typically 10-24 ng/dL
Conventional Interpretation: This test is rarely performed by conventional physicians, who see no value in this measurement.
Integrative Interpretation: Integrative physicians and those who some doctors who focus on optimal hormone balance consider elevated RT3 or an RT3/T3 ratio imbalance to be a key sign of an underactive or dysfunctional thyroid. They believe that the reverse T3 should fall in the lower half of the normal range.
 

TPO - Thyroid Peroxidase Antibodies / Antithyroid Peroxidase Antibodies

Measures: Antibodies that attack the thyroid gland, typically causing the destruction of the gland's ability to produce thyroid hormone.
Reference Range: Less than 35 IU/mL
Conventional Interpretation: Levels above the reference range are indicative of Hashimoto's Thyroiditis, an autoimmune disease. In the conventional view, however, Hashimoto's requires no treatment unless accompanied by overt hypothyroidism, as reflected by a significantly elevated TSH level.
Integrative Interpretation: Some endocrinologists and many integrative physicians feel that TPO antibodies above the reference range not only indicate Hashimoto's Thyroiditis, but that treatment -- even when other thyroid levels are normal -- may be warranted to resolve symptoms, prevent an increase in antibody levels, and/or prevent progression to overt hypothyroidism.
 

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Read Challenges to Getting a Thyroid Diagnosis for guidance on how to get properly diagnosed.

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