TSH (Thyrotropin) Test
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Thyroid Tests
Blood tests to measure low or high TSH (thyrotropin) levels are readily available and widely used. Not all thyroid tests are useful in all situations.
TSH (Thyrotropin) Test
The best way to initially test thyroid function is to measure the TSH (Thyroid Stimulating Hormone) level in a blood sample. Changes in TSH can serve as an "early warning system" — often occurring before the actual level of thyroid hormones in the body becomes too high or too low.
A high TSH level indicates that the thyroid gland is not making enough thyroid hormone (primary hypothyroidism). On the other hand, a low TSH level usually indicates that the thyroid is producing too much thyroid hormone (hyperthyroidism). Occasionally, a low TSH may result from an abnormality in the pituitary gland, which prevents it from making enough TSH to stimulate the thyroid (central hypothyroidism). In most healthy individuals, a normal TSH value means that the thyroid is functioning properly.
What is a T4 test?
A Total T4 test measures the bound and free thyroxine (T4) hormone in the blood. A Free T4 measures what is not bound and able to freely enter and affect the body tissues.
What does it mean if T4 levels are abnormal?
Importantly, Total T4 levels are affected by medications and medical conditions that change thyroid hormone binding proteins. Estrogen, oral contraceptive pills, pregnancy, liver disease, and hepatitis C virus infection are common causes of increased thyroid hormone binding proteins and will result in a high Total T4. Testosterone or androgens and anabolic steroids are common causes of decreased thyroid hormone binding proteins and will result in a low Total T4.
In some circumstances, like pregnancy, a person may have normal thyroid function but Total T4 levels outside of the normal reference range. Tests measuring free T4 – either a free T4 (FT4) or free T4 index (FTI) – may more accurately reflect how the thyroid gland is functioning in these circumstances. An endocrinologist can determine when thyroid disease is present in the context of abnormal thyroid binding proteins.
What is a T3 test?
T3 tests measure triiodothyronine (T3) levels in the blood. A Total T3 test measures the bound and free fractions of triiodothyronine. Hyperthyroid patients typically have an elevated Total T3 level. T3 tests can be used to support a diagnosis of hyperthyroidism and can determine the severity.
In some thyroid diseases, the proportions of T3 and T4 in the blood change and can provide diagnostic information. A pattern of increased T3 vs T4 is characteristic of Graves ’ disease. On the other hand, medications like steroids and amiodarone, and severe illness can decrease the amount of thyroid hormone the body converts from T4 to T3 (active form) resulting in a lower proportion of T3.
T3 levels fall late during hypothyroidism and therefore are not routinely used to evaluate patients with underactive or surgically absent thyroid glands.
Measurement of Free T3 is possible but is often not reliable and therefore may not be helpful.
What is a normal thyroid (hormone) level?
To assess thyroid hormone status for low or high TSH levels we use TSH and FT4 tests. The normal value for a laboratory test is determined by measuring the hormone in a large population of healthy individuals and finding the normal reference range. Normal ranges for thyroid tests may vary slightly among different laboratories, and typical ranges for common tests are given below.
TSH normal values are 0.5 to 5.0 mIU/L. Pregnancy, a history of thyroid cancer, history of pituitary gland disease, and older age are some situations when TSH is optimally maintained in a different range as guided by an endocrinologist.
FT4 normal values are 0.7 to 1.9ng/dL. Individuals taking medications that modify thyroid hormone metabolism and those with a history of thyroid cancer or pituitary disease may be optimally managed with a different normal FT4 range.
Total T4 and Total T3 levels measure bound and free thyroid hormone in the blood. These levels are influenced by many factors that affect protein levels in the body, including medications, sex hormones, and liver disease.
- A normal Total T4 level in adults ranges from 5.0 to 12.0μg/dL.
- A normal Total T3 level in adults ranges from 80-220 ng/dL.
Free T3 assays are often unreliable and not routinely used to assess thyroid function.
What does it mean if my thyroid levels are abnormal?
- Lab results: High TSH, low thyroid hormone level
Consider...Primary hypothyroidism - Lab results: High TSH, normal thyroid hormone level
Consider...Subclinical hypothyroidism - Lab results: Low TSH, high thyroid hormone level
Consider...Primary hyperthyroidism - Lab results: Low TSH, normal thyroid hormone level
Consider...Early or mild hyperthyroidism