Thyroid Stimulating Hormone
Thyroid Stimulating Hormone or TSH or Thyrotropin is a hormone produced by the pituitary gland, a small organ located below the brain and behind the sinus cavities. It is responsible for stimulating the thyroid gland. TSH levels are measured to diagnose any thyroid disorders.
The level of thyroid hormones in the blood is controlled by the pituitary gland and the TSH levels. The TSH is secreted by thyrotrope cells in the front pituitary gland which is responsible for regulating the endocrine function of the thyroid gland. The TSH level is directly related to the level of thyroid hormones.
The TSH is responsible for the secretion of the following hormones by the thyroid gland:
- Triiodothyronine or T3
- Thyroxine or T4
Ironically, T3 and T4 restrain the production and secretion of TSH. This is similar to another hormone somatostatin that inhibits TSH production.
The thyroid gland is located at the anterior part of the throat just below the Adam’s apple. This butterfly-shaped gland produces hormones that affect and regulate nearly every bodily function. Along with these, Triiodothyronine (T3) and Thyroxine (T4) are also necessary for the normal development of the human brain. In case of any thyroid disorders in the early years of a child resulting in insufficient thyroid hormone production, the child may become mentally retarded.
These hormones control various functions of the body including memory, heart rate, cholesterol levels, metabolism and organ function, weight loss or gain, energy levels and skin condition.
Thyroid Gland Disorders
When the thyroid gland is producing less thyroid hormone, the pituitary gland produces increased TSH levels to stimulate the thyroid forcing it to increase its production of thyroid hormones. This condition is called hypothyroidism. Symptoms of hypothyroidism include tiredness, dry skin, slow growth, constipation and delayed pubertal development.
When the thyroid gland produces more thyroid hormone than required, the pituitary gland decreases its production of TSH levels to decrease the level of thyroid hormones produced by the thyroid. This condition, hyperthyroidism, results in unexpected weight loss, irregular heartbeat, excessive sweating, irritability and nervousness.
Thyroid stimulating hormone test or TSH testing is an important part of diagnosing and treating any thyroid disorder. It also helps to assess the function of your pituitary gland. A TSH test may also be ordered in case of enlarged thyroid gland, hyperthyroidism or hypothyroidism. In addition to this, TSH tests are usually done for newborns to check any case of congenital hypothyroidism. Asymptomatic pregnant women are not tested generally but in case of any symptoms of thyroid disorder, the TSH blood levels are checked to detect and evaluate hyperthyroidism or hypothyroidism. It is usually done along with other thyroid tests like T4 test, T3 test and thyroid antibodies. This procedure is followed because the TSH is not the secretion of thyroid gland but of pituitary gland. Otherwise, if there is some problem with the pituitary gland itself, the test results may not be of any use.
TSH test is a common blood test used to detect any problems affecting the thyroid gland. A simple procedure, can provide an accurate reading of TSH levels. A blood sample is collected using a special lancet to break the skin and make a lesion on the nick of the finger. A few drops of blood are collected into a special collection device and sent for analysis. There’s no need to fast or limit any activity before the test. No special preparations are required for this test. However, because some medications may affect test results, tell the doctor about any drugs you are taking. Also if you had any radioactive substances or x-rays using iodine dye within 4-6 weeks your TSH test results may not be accurate.  Any kind of extreme stress and acute or chronic illness can also affect the TSH test results.
TSH TEST Results
Normal TSH Level
Though the TSH levels in the blood vary, the normal TSH levels for a newborn are generally 3-20mIU/L and for an adult 0.4-4.5mIU/L.
Normal ranges vary depending on the lab but are usually close to each other. Any value below 0.3mIU/L though is considered as hyperthyroid TSH level.
AACE set the new normal range to 0.3-3.0mIU/L a few years back while some doctors believe that TSH level over 2.0mIU/L is not normal and 3.5mIU/L is a hypothyroid TSH level.There is variation and disagreement about what the normal TSH levels are within particular populations. For example, in the United States it is considered to be normal if the TSH levels are between .5 to 4.5/5.0/ 
If the TSH level is blood is normal, then the other thyroid tests like T3 test and T4 test are carried out. The thyroid is considered to be functioning normally if the T3 and T4 levels are also normal.
High TSH Level
A high TSH level indicates that the pituitary gland is producing excess amounts of TSH and a disorder of thyroid gland exists. This condition is called hypothyroidism. TSH levels above 4.0mIU/L are considered to be high. This will happen if the pituitary gland makes more than the normal amount of TSH. That means the thyroid is not secreting enough T4 and T3. As a consequence the doctor will administer the T4 and T3 tests in addition to the TSH blood test.
If the TSH levels are high but the T4 and T3 tests show normal levels then it is considered as a case of mild hypothyroidism or sub-clinical hypothyroidism. Patients usually experience fatigue and mild depression.
If the T3 and T4 levels drop down considerably with a high TSH level then it is a condition of hypothyroidism. Patients may have
- memory loss,
- difficulty waking up,
- rough hair and skin,
- difficulty in losing or gaining weight,
- intolerance to cold,
- mood swings,
- abnormal menstruation and
- decreased sex drive.
During pregnancy, the thyroid levels change greatly and hence the hypothyroidism is often diagnosed. TSH levels during pregnancy are also necessarily determined because even sub-clinical hypothyroidism can cause pre-eclampsia, anemia, postpartum hemorrhage, fetal cardiac dysfunction or low birth weight. It even involves high risk of miscarriage or impaired development of brain function of the child.
Low TSH Levels
A low TSH level is an indication of either an overactive thyroid gland or damaged pituitary gland which is producing less Thyroid-stimulating hormone. This condition is called hyperthyroidism. It can also indicate an under-active thyroid gland which is receiving increased amount of TSH. The standard scale for determining the normal TSH levels vary around the world, but usually a TSH level below 0.3mIU/L is considered as a low TSH level.
The T3 and T4 tests may vary even when the TSH test shows low values. If the thyroid hormones tests are normal, then it may be a case of mild hyperthyroidism or sub-clinical hyperthyroidism.
If the T3 and T4 levels are high along with low levels of TSH, then it is an indication of increased hyperthyroidism. The symptoms for these include fatigue, palpitations, insomnia, nervousness, hair loss, muscle weakness, trembling limbs, increased bowel movements and weight loss.
In some cases of low TSH levels, the T3 and T4 tests also show low levels of the thyroid hormones. This indicates a non-thyroidal illness or a rare pituitary hypothyroidism.
If hypothyroidism is left untreated, it can cause myxedema coma, a dangerous condition that may result in loss of normal brain function and even death. Though, this is a rare condition, it is most common in women and aged hypothyroid patients whose thyroid condition is left untreated for a long period. It is commonly noticed during winter months and in extremely cold places.
- Clinical Chemistry Principles, Procedures, Correlations – Michael L. Bishop, Edward P. Fody and Larry E. Schoeff