TSH or thyroid stimulating hormone is a glycoprotein hormone which has a critical role in thyroid functioning. TSH regulates thyroid gland in synthesis and release of thyroid hormone, namely T3 and T4. Thyroid is an important gland situated in front portion of neck. The gland has variety of important functions to play in the body. It regulates metabolic activities, body temperature, and even has its influence on sex hormones.
TSH is produced by pituitary gland in brain. The role of TSH is to stimulate thyroid gland to produce T3 and T4 hormones when they are depleted in the circulation. When thyroid hormone reaches its normal level in blood, TSH secretion slows down. Low level of TSH can result either due to malfunctioning of thyroid gland or a problem related to pituitary gland.
What Causes Suppressed TSH?
As mentioned earlier, TSH hormone is suppressed either due to compromised thyroid function or conditions related to pituitary gland which actually manufactures TSH hormone.
- Hypopituitarism: It is a clinical condition characterized by suppression of TSH. Since pituitary gland located in the brain produces TSH, any problem with the pituitary gland or area around the gland in brain will suppress secretion of thyroid stimulating hormone (TSH). Pituitary tumor, radiotherapy, brain tumor, brain surgery, traumatic injury leading to skull fracture, brain infection such as meningitis and encephalitis, hemorrhage in brain all can cause hypopituitarism. Vascular problems resulting from Sheehan’s syndrome, snake bite, sickle cell disease can also cause hypopituitarism and suppression of TSH.
- Hyperthyroidism: The clinical condition results from excessive thyroid hormone T3 and T4 in blood circulation. Since production of both these hormones are controlled by TSH, when the level of T3 and T4 hormones become too high, the pituitary gland diminishes or suppresses or slows down the manufacturing of TSH just like a thermostat. TSH secretion is suppressed in Graves’s disease, the commonest type of hyperthyroidism. It is an autoimmune disease where the exact mechanism that initiates autoimmunity is not clear. However, the possible cause is suspected to be bacterial or viral agents. The antibodies produced stimulate production of thyroid hormone. TSH suppression can also occur due to thyroid nodules, drug induced hyperthyroidism as with the drug amiodarone. The symptoms of hyperthyroidism include weight loss, irritability, palpitation, increased sweating, tremors, dyspnoea, diarrhea, protrusion of eyeballs and heat intolerance.
- Pregnancy: During pregnancy a hormone called human gonadotrophin (HCG) is released by the placenta in the blood. HCG has some property in stimulating the thyroid to produce more T3 and T4 hormones. As a result TSH production is suppressed by the pituitary gland.
Long Term Effects Of TSH Suppression
Low or suppressed level of TSH can produce host of long term effects on the body. In the beginning patient may feel fatigued and restless. Patient is unable to bear heat, especially in summer season. Patient also perspires more than normal.
Palpitation of heart and erratic heartbeat are other important symptoms present in with low level of TSH. One of the heart related long term effect with suppression of TSH is enlargement of heart.
The hairs become thin and there is excess loss of hair. Skin becomes dark and warm. Loss of weight with increased appetite is characteristic feature of suppressed TSH. Patient has increased bowel movements and loose stools.
Normal Thyroid Stimulating Hormone Levels
To estimate normal value of TSH, the health professional withdraws blood from vein. The blood collected in a special tube is then analyzed with several laboratory methods. Normal value of TSH in adults is 0.4 – 4.2 micro units per milliliter (Mcu/ml).
However, a slight difference in normal range may vary from one laboratory report to another. Therefore it is always necessary to read the reference range of normal value given at the side of estimated TSH value.