The primary function of the thyroid is to produce thyroid hormones, thyroxine (T4) and triiodothyronine (T3). More than two thirds of T4 is converted to T3, which is more potent. T3, however, is a temporary hormone and lasts in the system for only a brief period.
The pituitary is the controlling gland for the thyroid gland. It acts on chemical signals received from the hypothalamus. Thyroid-stimulating hormone (TSH) is a peptide hormone secreted by the thyrotrope cells in the anterior pituitary gland. It produces or inhibits the production of TSH, also known as thyrotropin, and helps to maintain a healthy and constant level of thyroid hormones in the blood.
The system works as a regulatory feedback loop. When the levels of thyroid hormones thyroxine (T4) and triiodothyronine (T3) increase, the secretion of TSH is halted, and conversely when the levels are low, the secretion of TSH is increased.
The diagnostic use of TSH is limited to checking its levels for determining thyroid problems. Generally, a level of TSH between 0.4 and 5.0 uIU/mL (equivalent to mIU/L) is considered to be normal in adults.
Elevated levels of TSH are commonly associated with hypothyroidism, a thyroid condition, where the thyroid is not producing enough hormones to sustain adequate metabolism. The hypothalamus detects the inadequacy and signals the pituitary through thyroid release hormone (TRH) to produce more TSH. On the other hand, if there is too much of thyroid hormone in the blood, the hypothalamus produces somatostatin, which inhibits the release of TSH.
There are, however, conflicting views regarding elevated TSH in the blood. One of the reasons behind it is the fact that the human body is a complex system of action and reciprocal reactions. For example, the body will metabolize less thyroid hormone to preserve brain functioning in the event of starvation. Brain cells are a major target for thyroid hormones. Both T3 and T4 play an important role in the development of the brain, even during pregnancy. If a person is not eating enough, there is no way that the brain can signal for extra hormones to be sent to it to preserve brain functioning. The thyroid may keep on producing the same amount of hormones as before and the body may develop symptoms of low thyroid without affecting TSH level in the blood.
That indirectly means that elevated level of TSH by itself is not the only determining factor. The level of T3 and T4 in the blood has to be read along with the symptoms of the patient to properly interpret the state of thyroid health.