The endocrine system is a complex and interrelated system that produces endocrine secretions that help in controlling the metabolism in the body. Endocrine secretions are known as hormones, and these are transmitted by the blood to relevant tissues.
The thyroid-stimulating hormone (TSH) is produced by the pituitary gland located at the base of the brain. The hypothalamus in the brain produces a thyroid release hormone (TRH), which is transmitted to the anterior pituitary through the bloodstream to increase or decrease the release of TSH. The TSH stimulates the thyroid and initiates the production of the thyroid hormones.
Initially, thyroid problems were diagnosed by studying the basal metabolic rate, the rate at which the body burns energy when an individual is resting. Cholesterol levels and creatine phosphokinase were also studied to arrive at the typical thyroid conditions that may be ailing the body. With the passage of time newer tests became more prevalent. As radioiodine became available, its uptake by the thyroid was said to be the most reliable method of judging thyroid conditions. Later, protein bound iodine became the only necessary test. Testing the level of thyroxine was the next test that gained popularity. Ultimately, testing TSH levels in the blood became the ‘gold standard’ for diagnosing thyroid dysfunction.
The level of thyroid hormones, thyroxine (T4) and triiodothyronine, (T3) has a countervailing effect on the pituitary gland. High levels of thyroid hormones are a signal to the pituitary to release lesser TSH and the other way round. A high level of TSH is thus considered to be an indication of a thyroid condition known as hypothyroidism or an under-active thyroid.
This is actually a simplistic explanation and cannot be considered as being the final word on the subject. The fact is that diagnosing thyroid problems can be a pretty demanding job for a physician. Many times, people have all the symptoms of a particular thyroid condition but the blood levels of TSH are within the acceptable range. High levels of TSH do have an effect on the production of thyroid hormones, but only at extreme levels. TSH levels thus have to be read along with the symptoms of an individual. The level of free thyroxine also needs to be taken into consideration as almost 80% of it is converted to triiodothyronine. Sometimes the acceptable range of TSH level in the blood has to be recalculated.
To sum up, the effect of high level of TSH may be treated as an indication, but is not on its own a conclusive proof of under-active thyroid.