Testing for hypothyroidism
The diagnosis of hypothyroidism is currently almost entirely dependent on laboratory findings, with the TSH being seen by many endocrinologists
and GPs as the definitive test for thyroid assessment. The Free T4 is often requested only as a back-up test to confirm severe imbalances.
This has led to a situation where it is not unusual for the doctor to rely solely on the results of the TSH level to diagnose hypothyroidism.
The
endocrinologist Sir Richard Bayliss has stated “I have yet to see a hypothyroid patient without a raised TSH”.¹
This criterion for
diagnosis does not always include the clinical assessment of a patient’s symptoms, and as a result mild hypothyroidism can be missed.
Although
it is customary to describe a slightly increased TSH (i.e. over 4.5pmol/L) as mild or sub-clinical hypothyroidism, this is not always
treated. Certainly, a normal TSH with a borderline Free T4 is very rarely treated, even when the presenting symptoms may be typical
of hypothyroidism.
Normal ranges are usually based on total populations. However, when statistics suggest that from 7-10 per cent of
the female adult population may be hypothyroid; the accuracy of these normal ranges must be compromised.
This leads on to consider
the value of a closer correlation between patient’s symptoms and blood test results, even when the results fall within the “normal”
ranges. There are an increasing number of doctors, complementary practitioners and laboratory staff who consider that the normal ranges
used for thyroid testing in Britain are too broad. Furthermore there is evidence that the T4 normal range has become even lower and
the TSH has risen over the previous two years. In my own area of Dorset, the figures have changed as follows:
1996
1997
2007
TSH (mU/L)
Free T4 (pmol/L)
1995
Although it is recognised that different test procedures can lead to variations in the ranges, this does not fully explain why the British TSH and Free T4 ranges vary so much from many other countries. In particular the Free T4 normal ranges are very different than the UK figures, as in the following examples:
0.3 - 3.7
10.3-25.8
0.4 - 4
10.3 - 24.5
0.3 - 4.5
9.4 - 24
--
8-22
France Luxembourg Switzerland USA
12 – 28pmol/L
13 – 23pmol/L
13 – 30pmol/L
12 – 28pmol/L
Free T4
Some British hospitals and laboratories advocate even lower normal range figures e.g.7-18pmol/L.
Virtually the only medical treatment
for hypothyroidism is a prescription for thyroxine. This is currently a free prescription, for, as with the insulin prescriptions
for diabetes, thyroxine is seen as a life-long need.
Prescribing hormones usually leads to a dependency, as a result of the subsequent
reduced efficiency of the gland being treated.
It follows from this that a diagnosis of hypothyroidism should be viewed with some caution
as the treatment options are either thyroxine or no treatment at all. The need for an unequivocal diagnosis is reflected in the ever
widening normal ranges, leading to the current situation where only moderate or severe hypothyroidism is treated, and mild or sub-clinical
hypothyroidism frequently remains undetected and untreated.
The concept of a mild functional deficiency is well accepted in diabetes,
being termed late onset diabetes. The treatment is usually dietary advice, and insulin is rarely prescribed.
In America it is recognised
that many glands and organs can become functionally inefficient, causing mild symptoms, usually these symptoms develop before the
blood chemistry changes. This phenomena has been termed the Polyendocrine Syndrome. In Britain a more critical interpretation of blood
test results would lead to a better understanding of the ‘shades of grey’ that exist between normal and severe hypothyroid.
Again,
Sir Richard Bayliss has written “Certainly it is my experience that patients feel at their best when the free thyroxine level is towards
the upper end of the reference range, or marginally above it, and the TSH towards the lower limit of the normal range”.
Unfortunately
this view does not equate with the current diagnostic criterion of hypothyroidism.
Before the availability of laboratory evidence,
the diagnosis of hypothyroidism was based on clinical evidence or signs and symptoms. It may be rewarding to review the works of early
authors and to assess their value, with particular reference to mild hypothyroidism.
2
1
