The Limitations of Thyroid Stimulating Hormone Test
The thyroid stimulating hormone (TSH) test is a blood test which is often used to initially assess any thyroid dysfunctions. Until now, this is still the most reliable screening for any thyroid-related disorders because even if there are only small changes in hormone levels produced by the thyroid, the test produces large changes in thyroid stimulating hormone levels.
The close linkage between the pituitary and thyroid glands works like a feedback loop and acts as a thermostat. The thyroid stimulating hormone, a polypeptide with a molecular weight of approximately 10,000, is manufactured in your pituitary gland. It regulates the uptake of iodine and the synthesis of thyroxine by your thyroid gland. As a related effect, the levels of thyroid hormone in your blood control the secretion of TSH. Any increase or decrease in thyroid hormone levels triggers the pituitary to raise or lower its production of thyroid stimulating hormones accordingly.
Thyroid hormone levels also influence the hypothalamus cells responsible for producing thyrotropin-releasing hormone (TRH), which serves as secondary trigger in regulating thyroid stimulating hormone. You thus see the fine sensitivity of TSH to hormonal fluctuations of the thyroid. The TSH level will respond to the slightest deviations, long before you feel any symptoms.
However, TSH test have some limitations:
* The normal range of thyroid stimulating hormone is generally between 0.4 to 5.5 microunits per millilitre (mU/mL) of blood. This range varies slightly among laboratories, however, because their normal values use measurements taken from large numbers of people with healthy thyroids. Since doctors treat patients as individuals (not groups), the values should not be indiscriminately applied on every patient.
When an individual’s TSH level is tested repeatedly, the readings should vary by about 0.5 mU/mL. Thus, if you have a TSH of 1.0 most of the time, the range of variation during successive tests will be 1.0 plus or minus 0.5 (or 0.5 to 1.5) mU/mL.
But if your TSH level is on the extreme ends of normal range, the variation should not be so great that it switches to the other extreme during successive testing. When that happens, although the readings may fall within normal range, it may point to thyroid disease.
* The original values for normal range may actually be a little off. Recent studies suggest that at the time the normal TSH range was generated (about half a century ago), some participants in the tests may have been mildly hypothyroid and mildly hyperthyroid, which made the range wider.
Therefore, a number of professional endocrine societies are calling for a narrower normal range, especially on the higher end. The upper limit will be lowered from 5.5 to 4.2, which will enable physicians to screen out more people with true thyroid dysfunctions, who are on the fringes of the current range.
In examining patients without any symptoms, the thyroid stimulating hormone test that yields normal results would virtually rule out thyroid disease. But among patients with symptoms that suggest some thyroid dysfunction, the TSH test should only be the first step to diagnosis. The doctor should never come to conclusions based on a single number.
