The sensitive thyroid stimulating hormone (TSH) test is generally considered to provide the most accurate measure of the appropriateness of thyroid gland activity. In the past, physicians were unable to detect thyroid hormone excess or deficiency until a patient’s symptoms were fairly advanced. With the sensitive TSH test, however, physicians are able to diagnose thyroid disorders at an earlier stage -- in many cases, even before patients begin to experience symptoms.

TSH tests, due to their high degree of sensitivity, enable physicians to detect even the mildest abnormalities of thyroid function. Early detection and treatment of thyroid disorder allows physicians to prevent the onset of symptoms and long-term complications.

How the Thyroid Functions
The thyroid gland operates as part of a feedback mechanism involving the hypothalamus and the pituitary gland. First, the hypothalamus sends a signal to the pituitary gland through a hormone called TRH (thyrotropin releasing hormone). When the pituitary gland receives this signal, it releases TSH (thyroid stimulating hormone) into the bloodstream, where it is carried to the thyroid gland. Upon receiving TSH, the thyroid responds by releasing two hormones, thyroxine (T4) and triodothyronine (T3), which then enter the bloodstream and affect the metabolism of the heart, liver, muscles and other organs. Finally, the pituitary “monitors” the level of thyroid hormone in the blood and increases or decreases the amount of TSH released, which then regulates the amount of thyroid hormone in the blood.

A “Measurable” Improvement in Thyroid Tests

Prior to the introduction of the TSH test, the standard blood tests for thyroid disorders measured only T4 and T3 levels in the blood. Research has shown, however, that these hormone levels can appear relatively normal even when the thyroid is not functioning normally, because the pituitary gland will under- or over-compensate for thyroid function. When the thyroid gland begins to fail (leading to hypothyroidism or an underactive thyroid), the pituitary gland produces higher levels of TSH in an attempt to simulate the gland to produce more T4 and T3. On the other hand, when the thyroid gland begins to produce too much thyroid hormone (seen with hyperthyroidism or an overactive thyroid), the pituitary gland decreases or completely stops producing TSH.

TSH testing offers a marked improvement over simply measuring T4 and T3 by taking advantage of the “wisdom” of the pituitary gland, the organ that first recognizes thyroid dysfunction, and therefore provides a more exact measure of thyroid function.

Beyond disease diagnosis, TSH testing plays a critical role in helping physicians manage thyroid disorders. In this capacity, TSH tests are used to determine the optimal medication dosages for patients with hypothyroidism and hyperthyroidism, as well as to monitor changes in dosage requirements over time.

Interpreting a TSH Test
A TSH test is done by drawing a small blood sample and sending it to a laboratory for analysis. The laboratory will measure the level of TSH, and based on a reported normal value range, it will determine whether the level is below normal (hyperthyroid), above normal (hypothyroid) or within the normal range (euthyroid).

People who believe they may have a thyroid disorder should ask their physician to have their TSH level checked. Ultimately, periodic screening may lead to early recognition and treatment of thyroid dysfunction. Without treatment, mild thyroid failure may develop into overt hypothyroidism at a rate of 5 to 26 percent per year. Furthermore, mild thyroid underactivity, while not necessarily producing any symptoms, may be associated with increased cholesterol levels. Sensitive TSH tests, with their unique ability to detect mild thyroid failure, can help prevent this undesirable result. Another reason why TSH testing is more accurate than simply measuring T4 and T3 is that most T3 is actually made in peripheral tissues rather than being directly secreted by the thyroid and the body can preferentially make T3 even if the thyroid is not working as well as it should. This is why a person who is hypothyroid can take only thyroxine (T4) and still make adequate amounts of T3 for proper body function.

In November 2002, AACE published revised guidelines that provided a new range for normal thyroid function. Until this time, doctors had relied on a normal TSH level ranging from 0.5 to 5.0 to diagnose and treat patients with a thyroid disorder who tested outside the boundaries of that range. AACE encouraged doctors to consider treatment for patients who test outside the boundaries of a narrower margin based on a target TSH level of 0.3 to 3.04. AACE believes the new range will result in improved accuracy of diagnosis for millions of Americans who suffer from a mild thyroid disorder, but have gone untreated.

In order to develop the new range, an AACE task force reviewed available data and current trends in practice, and updated the AACE Guidelines for Clinical Practice for the Evaluation and Treatment of Hyperthyroidism and Hypothyroidism. The task force also suggested that, in patients on thyroxine replacement therapy, "The target TSH level should be between 0.3 and 3.0 uIU/mL." By inference, any patient with TSH outside this range has an "abnormal" level which should be carefully followed but not necessarily treated. Some authors, however, treat marginally abnormal TSH when coupled with positive antithyroid antibody titers because of high progression rates to clinical hypothyroidism. This area remains very controversial but other endocrine organizations such as The Endocrine Society and the American Thyroid Association are in agreement with the AACE position.

When to See an Endocrinologist
AACE recommends hypothyroid patients in the following categories should see an endocrinologist:

  • Patients of age 18 years or less
  • Patients unresponsive to therapy
  • Pregnant patients
  • Cardiac patients
  • Presence of goiter, nodule, or other structural changes in the thyroid gland
  • Presence of other endocrine disease
  • In support of this precise tool for measuring thyroid activity, the American Association of Clinical Endocrinologists (AACE) clinical practice guidelines strongly advocate the use of TSH tests for diagnosing thyroid conditions. For more information about TSH testing or thyroid disorders, visit the AACE web site at or