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Thyroxine - a suitable case for treatment

One of the most useful developments in laboratory diagnostics in recent years has been the advent of new high-sensitivity assays for thyroid stimulating hormone (TSH). It is an important test for many aspects of pituitary and thyroid function, but it is becoming much more useful as the sensitive assays can now define a reference (or normal) range of about 0.4 to 4.0 mU/L. Values below normal can now be measured (in some systems down to 0.002 mU/L), whereas previous methods could not discriminate values below normal.

TSH and thyroxine treatment

TSH measurement is useful in measuring whether patients who receive thyroxine for hypothyroidism are getting the right dose; TSH is elevated in untreated patients, but falls to the normal range in a few weeks of treatment with an appropriate dose. That is the theory, but until recently, this could not be tested in practice.

Now, however, a group in Birmingham has reviewed the status of treatment of patients with hypothyroidism in general practice in England. They examined four West Midlands general practices who had 18,944 patients registered. Of these, 146 (0.8%) were being prescribed thyroxine, 134 for primary hypothyroidism, and the remainder for other appropriate reasons, such as hypopituitarism and thyroid cancer.

48% of hypothyroid patients get the wrong dose

Or, of course, they may not take it if given the right dose. In any event, as judged by the serum TSH, 27% were being undertreated as they had high TSH levels, and 21% were being overtreated, as they had low TSH values. The proportion of undertreated patients was high in those with the lowest doses of thyroxine.

Is this a bad thing? The truth is that we probably don't know the full effects of under or over treatment, as up till now there have of necessity been no studies. Half of those undertreated in this survey (about 13% of all patients with treated primary hypothyroidism) had TSH values above 10 mU/L; it is unlikely that all the symptoms of hypothyroidism would be completely eliminated in these patients. Undertreatment may be associated with increased long-term risks of heart disease, but this is not quantifiable. Overtreatment is still more difficult - but osteoporosis may be a potential problem.

What of the future?

Methods for measuring TSH accurately within and below the normal range are now commonly available and should be accessible to Trusts and GPs. Regular biochemical monitoring for TSH (say annually) of patients with primary hypothyroidism treated with thyroxine should be useful for encouraging compliance. Further research is indicated to determine the benefits that might accrue from maintaining appropriate therapy in this group of patients.

Reference:

Parle et al, British Journal of General Practice 1993 43: 107-9.

Questions to be Answered
Q: What need is met by this test?
A: TSH is used in the diagnosis of thyroid disease; new assays with high sensitivity also allow it to be used to monitor the effectiveness of thyroid replacement.
Q: What happens at present?
A: Surveys show that patients on thyroxine have TSH checked irregularly - and that 48% are taking the wrong dose. This is 0.4% of the population.
Q: Is quality improved?
A: Not proven for the whole thyroxine relacement population, though it is certain that quality will be improved for a significant percentage of these patients.
Q: What is the cost?
A: Most laboratories perform thousands of tests a year. There is a need to ensure that they have adequate sensitivity, but the incremental cost of testing thyroxine replacement patients annually is minimal.
Q: Can cost savings be made?
A: Probably not in any volume.
Advice to Health Authorities and GPFHs
  • Will increase quality and effectiveness
  • Cost neutral
  • Worth considering including need for high sensitivity TSH tests in specifications to laboratories.
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