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Thyroid Blood Test

A simple blood test helps measure the levels of thyroid hormones. Abnormal thyroid hormone levels can lead to weight loss, fatigue and weakness. Scroll down to know thyroid blood test normal range...
A simple blood test that helps detect thyroid disorders like hypothyroidism and hyperthyroidism is known as thyroid blood test. You may find the test results a little confusing as they include numbers indicating T3, T4 or TSH levels. If you are confused about the interpretation of those numbers, then this article will help you understand what do T3, T4 and TSH indicate and how are they interconnected.

Blood Test to Measure Thyroid Hormones

If symptoms of affected thyroid levels such as weight loss without any reason, increased heart rate, increased bowel movement, heat intolerance, palpitations, arrhythmia, excessive sweating, increased irritability, decreased concentration, tremors, insomnia, depression, etc. are noticed, then to confirm the diagnosis, doctors generally suggest a blood test. Those diagnosed with thyroid problems also need to undergo a blood test to monitor the effectiveness of the treatment. The test that helps measure thyroid hormone levels also helps detect causes of infertility in women. Since the hormone released by the pituitary gland stimulates the secretion of thyroid hormones, thyroid hormone levels are also helpful in evaluating pituitary disorders. This hormone released by the pituitary gland is known as thyroid stimulating hormone or TSH.

Scarcity of thyroid hormones due to an underactive thyroid (low thyroid levels) and excessive production of thyroid hormones due to an overactive thyroid (high thyroid levels), can affect your health significantly. Various diseases can cause abnormal thyroid levels, for example, pituitary disorders, autoimmune diseases like Hashimoto's thyroiditis, etc. Hypothyroidism is one of the common iodine deficiency symptoms.

T3, T4 and TSH

Thyroid gland is responsible for the production of the hormone triiodothyronine (known as T3) and thyroxine hormone (known as T4). As mentioned earlier, release of these two hormones is regulated by TSH. So a TSH blood test is performed to measure TSH levels which also give an idea about T3 and T4 levels. When T3 and T4 levels are elevated, pituitary gland releases less TSH. If T3 and T4 levels drop down due to an underactive thyroid, then pituitary gland will release more TSH. Thus TSH level is inversely proportional to T3 and T4 levels.

Normal Thyroid Levels Chart

Thyroid Level Test Normal Range (mIU/L) Interpretation
T3 (Serum triiodothyronine) 80 - 220 T3 < 80 indicates hypothyroidism.
Free T3 2.3 to 4.2 Free T3 < 2.3 suggests hypothyroidism.
Total T4 (Serum thyroxine) 4.5 to 12.5 T4 < 4.5 with elevated TSH indicates underactive thyroid. T4 > 12.5 indicates hyperthyroidism. T4 < 4.5 with low TSH suggests a pituitary problem.
Free T4 0.7 to 2.0 Free T4 < 0.7 suggests hypothyroidism.
TSH Test 0.3 to 3.0 (as of 2003, revised by the American Association of Clinical Endocrinologists) but many labs or doctors follow the old numbers 0.4 to 6 TSH < 0.3 indicate hyperthyroidism and TSH > 3.0 indicate hypothyroidism.

Thyroid blood test cost may vary from lab to lab or from state to state. It is a simple blood test but you are expected to inform your doctor about your routine medications. Certain medicines like aspirin can affect thyroid levels. If you have thyroid level problems and if you are undergoing a blood test to monitor the effectiveness of thyroid hormone replacement therapy, then also it is necessary to inform your doctor about your routine medication, before the test. Low thyroid levels during the first trimester of pregnancy is very common because thyroid levels are significantly influenced by stress. Certain illnesses can also affect the levels of thyroid hormones.

It should be borne in mind that thyroid levels are not constant throughout the day. For correct interpretation of your reports, you should consult your doctor.
By Leena Palande
Published: 10/27/2010
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