There are a number of different blood tests that can be performed to test whether the thyroid is functioning correctly, but it’s the baseline tests of three critical hormones – T4, T3 and TSH – that usually kickstart a diagnosis.
The thyroid hormones are produced on a signal from the pituitary gland in the brain. This gland releases TSH (thyroid stimulating hormone), which in turn causes the thyroid to release T4 (Thyroxine) and T3 (Triiodothyronine), the two hormones that regulate the body’s metabolism at the cellular level. If the balance of these hormones is disrupted for any reason, the metabolic rate can be significantly raised (as in hyperthyroidism) or lowered (as in hypothyroidism). In Hashimoto’s disease (an autoimmune form of thyroiditis), sufferers can experience both of these conditions within the same day or week.
TSH Blood Levels
TSH is thyroid stimulating hormone, the hormone released by the pituitary gland. As this is a vital part of the feedback system (regulating the amount of thyroid hormones that the body will produce), levels of TSH indicate not only a potential thyroid problem, but also any problems within the pituitary gland. The good news is that a “normal” reading on this blood test normally indicates that the thyroid is healthy and functioning normally.
The normal range of a serum thyrotropin is considered to be from around 0.5–4 mIU/L. This blood test is always run in conjunction with T4 and T3 tests (see below); but as a guideline:
- If the TSH level is above normal, the person tested may be hypothyroid
- If the TSH level is over 2.0 mIU/L but T4 levels are normal, the person tested would be considered to have subclinical hypothyroidism. This means blood tests should be performed every 6–12 months, as hypothryoidism is likely to develop in the future.
- If the TSH level is below normal, the person tested may be hyperthyroid
- If the TSH level is very low, it may be because the pituitary gland is trying to compensate for excess T3 and T4 hormones in the blood caused by Graves disease. This is because antibody thyroid-stimulating immunoglobulins (TSI) are mistakenly instructing the thyroid to make too much T3 and T4.
Measuring T4 (Thyroxine) Levels in the Blood
When the body releases thyroid hormones, it releases both thyroxine (T4) and triiodothyronine (T3); in a ratio of around 80:20. Thyroxine (T4) is then converted to the active hormone (T3), mainly by the liver. The blood tests reflect the amount of these hormones in the blood.
The T4 test (also called Total T4, Total Thyroxine, or Serum Thyroxine test), has a normal range of approximately 4.6–12 ug/dl. Today doctors often measure Thyroxine in its free form (Free T4), because some T4 is bound to proteins and unavailable to the body; Free T4 is free and able to enter the various target tissues in the body to exert its effects. The normal range of Free T4 is approximately 0.7–1.9 ng/dl.
- A low level T4, and a high TSH, is considered indicative of primary hypothyroidism due to disease in the thyroid gland
- A low level T4 and a low TSH usually indicates hypothyroidism caused by a problem with the pituitary gland. The thyroid is reacting appropriately, because it is producing low levels of T4 in accordance with low levels of TSH; but the pituitary gland is producing inadequate levels of its own TSH hormone. This requires immediate investigation, as the pituitary gland affects many the successful operation of many of the body’s other glands such as the adrenals, ovaries and kidneys.
- A high level T4 with a low TSH is indicative of hyperthyroidism.
- A low level T4 with normal TSH may indicate an excess of deficiency of Thyroid Binding Globulin, a type of protein in the blood that most thyroid hormones attach to. This does not cause problems beyond falsely raising or lowering the T4 measurements; actual levels of hormones are usually adequate, and no treatment is necessary. People with this (often hereditary) condition may be mistakenly diagnosed as being hyper- or hypothyroid.
Measuring T3 in Blood Tests
Doctors measure T3 particularly in cases where hyperthryoidism is suspected, as the T3 level will be obviously raised in these cases. A T3 blood test is not very useful in cases of hypothyroidism, as the T3 levels are the last to be affected – it’s possible to be severely hypothyroid, with a low T4, high TSH but utterly normal T3 level.
- This test is known as a Total T3 (T3) or Serum Triiodothyronine test, and the normal range is approximately 80-180 ng/dl.
- A high level T3 is normally indicative of hyperthyroidism, the most common cause of which is Graves’ Disease.
- However, a high reading of T3 accompanied by symptoms of hypothyroidism (cold, slow, depressed and constipated) might indicate Reverse T3 syndrome, and some doctors will order a Reverse T3 test. (read more about Reverse T3)
- A low level (less than 80), usually indicates hypothyroidism.
As with T4, T3 may be tested for levels of the Free form (Free T3, FT3, or Free Triiodothyronine l), for which the normal range is approximately 230–619 pg/d. A level of less than 230 might indicate hypothyroidism.
Note also that certain drugs, such as contraceptive birth pills and large doses of aspirin, can interfere with T3 test results. The hormone is also affected by illness; serious illness will reduce levels of T3 significantly.
Rebalancing the Thyroid Naturally
References:
"Diagnosing Hypothyroidism", University of Maryland Medical Centre
"Thyroid Gland Function"; James Norman MD, Endocrine Web
"The Thyroid Gland and Thyroid Cancer, NHS UK
Sarah Tomley - Sarah is an editor and writer who has been working in the UK publishing industry for over 18 years. She has an honours degree in ...
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