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Understanding the TSH Hormone

The Function of Thyroid Stimulating Hormone

Jul 26, 2009 Jim Lowrance

The pituitary-gland hormone called "Thyroid Stimulating Hormone" (TSH) is highly sensitive in detecting thyroid hormone imbalances earlier than any other blood test.

Thyroid Stimulating Hormone (TSH) is the most commonly used blood lab-tested hormone level in evaluating thyroid function. It is highly sensitive and able to detect thyroid hormone imbalances earlier than any other blood test. There are other interesting facts about this important endocrine hormone that will be addressed in the following subheadings.

TSH is not a Thyroid Hormone

TSH actually comes from one of the master brain-glands called the “pituitary gland”. It is the hormone that stimulates thyroid hormone production. The hormones released from the thyroid gland (T4 and T3) once responding to stimulation by TSH are the ones the regulate bodily metabolism. As the need for thyroid hormone fluctuates during a 24 hour period, the TSH level can change by as much as 2-points but will stay within normal values if there is not a problem within the thyroid gland.

TSH Rises when the Thyroid Gland is Under-Functioning

If the pituitary gland senses that there is not enough thyroid hormone being released (hypothyroidism) due to the gland struggling or being hindered by a disease process, it will send an excess of TSH to further stimulate thyroid hormone production. This is the point at which lab values of blood-tested TSH levels will usually be flagged high and will continue to rise as the hypothyroidism worsens. The hormone level will be sensitive enough, so that even mild, sub-clinical cases of hypothyroidism will be detected early into the development of them. Normal values range between blood testing labs but an approximate average of the low-normal range for TSH is between 0.3 and 0.5 and the high-normal range averages between 3.0 and 5.0.

TSH Falls with an Over-Functioning Thyroid Gland

The opposite effect will occur when the thyroid gland is producing too much thyroid hormone (hyperthyroidism). The pituitary gland will back-off and send less of the stimulating hormone when the thyroid is overactive. This is the pituitary gland’s attempt to prevent worsening of the hyperthyroidism and if overproduction of thyroid hormones is severe, TSH may actually become undetectable when blood testing the level. Both a low-normal TSH and a high-normal TSH or borderline levels on either end of the normal values merits follow-up blood retesting because sub-normal levels can be an indication of a developing thyroid hormone disorder.

TSH does not Detect Thyroid Disease

This amazing endocrine hormone, with its great sensitivity, actually does not diagnose the “cause” of an over-functioning or an under-functioning thyroid gland. Blood levels of TSH are valuable in detecting thyroid hormone imbalances but further testing is required to find the cause of a thyroid hormone imbalance. A disease process in the thyroid gland will most often be the cause of an abnormal TSH reading. There are however, other possible causes of TSH imbalance, including a tumor or disease process occurring within the pituitary gland itself and thyroid hormone imbalances due to a medication a person is taking or another disease process in the body not directly related to the thyroid gland. These types of causes affecting thyroid hormone levels are very rare compared to thyroid diseases.

Follow up blood testing of other thyroid-related levels may help to determine the cause of an abnormal TSH level and thyroid hormone imbalances. This includes testing for “thyroid antibodies” that can detect autoimmunity in the gland (immune system response) and testing the actual T3 and T4 thyroid hormone levels. Thyroid disease cases involving goiters or thyroid nodules may also require imaging tests or tissue biopsies.

MedlinePlus Medical Encyclopedia: TSH

TSH: Sources

The copyright of the article Understanding the TSH Hormone in General Medicine is owned by Jim Lowrance. Permission to republish Understanding the TSH Hormone in print or online must be granted by the author in writing.

Pituitary Gland

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Comments

Nov 30, 2009 7:25 PM

Guest :

what about when the T4&T3 are normal and TSH level about 7. CHANDRAN

Nov 30, 2009 8:07 PM

Jim Lowrance :

Hypothyroidism can develop before the T3 and/or T4 fall below normal values. Some people see TSH rise to 10. and 15. and thyroid hormones are still within normal range.

A "7.0" TSH is elevated according to all lab ranges for it that I have seen because highest normal averages about "5.0" at many labs but can be at 6.0 at a few of them. Some labs place highest-normal TSH at 4.0 and some experts even believe readings above 3.0 indicate developing hypothyroidism.

In my opinion a 7.0 reading merits follow up every few months by repeat blood testing despite normal T3 and T4 levels..

2 Comments