The most common questions that Thyroid Australia is asked involve the interpretation of Thyroid Function Tests (TFT’s). Many people have been told that their TFT results are ‘normal’. So what is ‘normal’? In this article we will focus on the test for Thyroid Stimulating Hormone (TSH) which is the most common test ordered.

The ‘normal’ Reference Range for the test is intended to represent the range of values which can be expected in the healthy population – ie those without any thyroid ailment. The Reference Range is found by taking a sample population of healthy individuals and determining their TSH levels. The lowest and highest 2.5% of readings are excluded so that the Reference Range covers 95% of the healthy population. There are a number of different tests for TSH with different levels of sensitivity. They each have their own Reference Range. The most common tests generally have lower limits to their Reference Ranges around 0.2 to 0.5 mIU/L and upper limits from 3.5 to 5.0 mIU/L.

A recent study in Norway provides a good example of the use of the TSH test in practice.¹

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The results for women are shown in the chart. The results for men were only slightly different.

The features of this result are:

• The distribution of TSH readings in the healthy population is skew. It is not the common bell shaped curve centred in the middle of the reference range.
• The most common value, or Mode, is at 1.25.
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The conclusions which can be drawn from this survey are:

• TSH results in the upper half of the Reference Range have a low probability of being ‘normal’. This does not mean that they are not ‘normal’. It means that they are unlikely to be ‘normal’.
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Another important point which needs to be borne in mind when interpreting statistics like these is that it is the population which has a range of values with probabilities for each reading. Each healthy individual is only at one of the points. They are ‘normal’ when they are at that point. For those on thyroxine replacement, being in the Reference Range is not good enough in itself. You need to be at your own set point. This will probably be near the lower end of the Reference Range.

This analysis of the distribution of TSH readings in the healthy population supports our recommendations to thyroid patients:

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• Do not accept that a Thyroid Function Test is 'normal' just because the result is within the Reference Range if you are still feeling unwell.

References

1. T Bjøro et al, 'Prevalence of thyroid disease, thyroid dysfunction and thyroid peroxidase antibodies in a large, unselected population. The Health Study of Nord-Trøndelag (HUNT).' European Journal of Endocrinology 2000 143 639-647. Download here.
2. J Stockigt, 'Subclinical Hypothyroidism or Mild Thyroid Failure: How important is early diagnosis and what treatment is optimal?' Interview With Sigma Pharmaceutical 2001 http://www.thyroid.org.au/Information/Stockigt.html. Accessed 30 December 2001.

Alun Stevens is an actuary with his own consultancy. He is also Secretary of Thyroid Australia

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