Thyroid gland is an endocrine gland which secretes two types of hormones, T3 and T4. These hormones are concerned with the regulation of metabolism in human body. The under activity of these hormones results in hypothyroidism while hyperthyroidism, is an outcome of over-activity of T3 and T4 hormones. The levels of T3 and T4 in the body are determined by another hormone called Thyroid Stimulating Hormone (TSH). This hormone is secreted by the pituitary gland. As the name suggests, TSH stimulates the thyroid gland to secrete T3 and T4. Hence, when the level of TSH increases, it directly implies that the thyroid gland is not functioning properly. On the other hand, when the level of TSH falls down below normal level, it is an indication of hyperactivity of thyroid gland.
Normal TSH Level
The normal thyroid level for TSH for labs in UK is 0.5 to 5.5 mIU/L (milli-international units per liter). Labs in the U.S. followed the same reference scale until 2003. In spring 2003, the scale was revised to new standards, which specified 0.3 to 3.3 mIU/L as normal TSH level. However, some labs still adhere to the old reference scale, resulting in non uniformity in treatment. Besides, there are many fluctuations in the TSH levels throughout the day. The TSH levels at 6 a.m. and 6 p.m. are approximately the same. There is a sharp increase in the TSH level in the evening. The level continues to increase until midnight, when highest TSH level is recorded. The TSH level falls down considerably around noon. The difference in the TSH levels throughout the day is about 2 - 3 points.
Normal T3 Level
Triiodothyronine or T3 is present in the body in very small amounts. However, it is very important from the point of view of regulation of metabolism. Normal thyroid hormone level for T3 depends upon the type of test to be conducted. If your physician has asked you to undergo a test called 'Total T3', you should expect the normal range to be between 80 to 220 points. Thyroid level below 80 indicates underactive thyroid or hypothyroidism, while high thyroid levels, i.e. above 220 indicate overactive thyroid or hyperthyroidism. Normal thyroid levels for yet another test called 'Free T3' fall in the range of 2.3 to 4.2 pg/ml. Thyroid levels below the lower limit and above the upper limit indicate abnormal thyroid activity.
Normal T4 Level
Thyroxine or T4 is transported in the bloodstream by proteins. It gets converted in T3 inside the liver. Again, for determining the level of T4, two types of tests are to be conducted. Normal thyroid levels for 'Total T4' are between 4.5 to 12.5 pg/ml. While, normal levels for 'Free T4' are within the range of 0.7 to 2.0 pg/ml. If your thyroid levels do not fall within the normal range, your physician may diagnose you for hypothyroidism or hyperthyroidism, depending upon your thyroid hormone level.
The following table summarizes the above information on normal range of thyroid hormones.
|TSH||0.3 to 3.0 mIU/L|
|Total T3||80 to 220 points|
|Free T3s||2.3 to 4.2 pg/ml|
|Total T4||4.5 to 12.5 pg/ml|
|Free T4||0.7 to 2.0 pg/ml|
Many a time, diagnosis is done by monitoring the TSH levels alone, without checking the T3 and T4 thyroid hormone levels. This is particularly risky as a person could be suffering from thyroid problems in spite of having normal TSH levels. On the other hand, TSH levels skewed from the normal range do not necessarily imply that the person is suffering from thyroid dysfunction. This is because TSH is a secretion of pituitary gland and not thyroid gland itself. Abnormalities with pituitary gland may cause TSH levels to fluctuate, in spite of having a perfectly functioning thyroid gland. Hence, it is necessary that all the three hormone levels be tested for proper diagnosis of thyroid function.
Antibodies test should be conducted in case a person has normal thyroid levels and yet exhibits thyroid dysfunction symptoms. Besides, a person may have his own set of thyroid levels that are normal for him, which makes the interpretation of test results even more complex.