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:
1995 1996 1997 2007
TSH (mU/L) 0.3 – 3.7 0.4 – 4 0.3 – 4.5 --
Free T4 (pmol/L) 10.3-25.8 10.3-24.5 9.4-24 8-22
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:
France 12 – 28pmol/L
Luxembourg 13 – 23pmol/L
Switzerland 13 – 30pmol/L
USA 12 – 28pmol/L
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.² 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”.¹
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.