Is There a Place for the TRH Stimulation Test?

Lab technician with blood samples and medical chart
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There are several different thyroid tests that can be used to see how your thyroid is functioning. One of them is the thyrotropin-releasing hormone (TRH) stimulation test, which used to be the preferred test, but is now rarely used. This test has gained some renewed interest from certain practitioners as a way to detect mild or subclinical hypothyroidism that's missed by the more common thyroid stimulating hormone (TSH) test.

However, the TRH test is considered outmoded by most practitioners, who consider the TSH test to be more sensitive and accurate than the TRH. Still, a small number of practitioners continue to use the TRH test to catch those patients who appear "normal" according to the TSH test.

Understanding the TRH Test

To understand how the TRH stimulation test works, it's first helpful to quickly review how the various brain hormones interact with the thyroid:

  • Your hypothalamus is a gland in your brain that secretes thyrotropin-releasing hormone (TRH).
  • When TRH is released, it stimulates your brain's pituitary gland to release thyroid stimulating hormone (TSH).
  • TSH stimulates the thyroid itself to make thyroid hormones.

Most practitioners use the TSH test, which measures the circulating level of TSH in the bloodstream at one point in time. The TSH test results are then interpreted; if your levels are higher or lower than the reference range, this is considered evidence of potential thyroid disease.

The TRH test is different. A baseline TSH test is done. Then you're given an injection of TRH, which stimulates your pituitary to release TSH. A second blood sample is drawn 20 to 30 minutes later, and the TSH level is retested. TRH is known generically as protirelin (Thyrel TRH).

Some practitioners feel that the TRH test can detect subtle thyroid problems and measure your thyroid's ability to respond in real time, compared to the TSH test, which is a snapshot of thyroid function at one point in time.

How the TRH Test Differs From the TSH Test

Comparing the TRH stimulation test and the TSH test is much like a cardiac stress test compared to a cardiogram, or a glucose tolerance test compared to fasting glucose level. In a stimulation test, the challenge may reveal an impairment in the thyroid.

Still, most doctors consider the TSH test highly accurate with only one blood draw required and no special supplies, plus it's inexpensive. In comparison, the labor-intensive TRH test requires two separate draws a half hour apart, the availability of protirelin, and knowledge of how to accurately perform and interpret the test. It also costs more.

Benefits of the TRH Test

The TRH test is occasionally used to help identify secondary hypothyroidism (hypothyroidism that's due to pituitary problems) and tertiary hypothyroidism (hypothyroidism that's due to hypothalamic disorder). It can also detect hypothyroidism earlier than the TSH test. However, there are only a few doctors in the U.S. who know how to use the TRH stimulation test. These doctors believe the test is important when traditional TSH tests results are borderline, and when a patient has obvious thyroid symptoms but normal TSH results.

Ultimately, researchers have shown that the TRH stimulation test can detect dysfunction in the thyroid, despite otherwise normal TSH and other thyroid blood test levels.

The TRH stimulation test can also reveal what's referred to as early sub-biochemical hypothyroidism before it's reflected in the more common TSH test. Hopefully, new studies will be conducted to determine the potential benefits to patients of more widespread use of the test since the most recent study on this topic is from 2007.

Cautions

If you are able to find a practitioner who does do the TRH stimulating test, keep in mind that the test should be used with caution if you have any of the following:

  • Asthma
  • Chronic obstructive pulmonary disease
  • Any form of heart disease caused by inadequate blood flow to the heart
  • Evidence of reduced activity of the pituitary gland (hypopituitarism)
  • Pregnancy 

Sources:

Doi SAR, Issac D, Abalkhail S, Al-Qudhaiby MM, Hafez MF, Al-Shoumer KAS. TRH Stimulation When Basal TSH is Within the Normal Range: Is There “Sub-Biochemical” Hypothyroidism? Clinical Medicine & Research. 2007;5(3):145-148. doi:10.3121/cmr.2007.756.

Kellman R. TRH Stimulation— The Rarely Done Thyroid Test You Need. Thyroid Nation. Published August 19, 2016.