Following on from recent mailbase discussions regarding advice given by
the Poisons Centre on thyroxine overdose, this week our A&E Dept.
received the following advice from Guy's (according to our A&E medical
staff):

Patient was not on T4 and had taken 'a handful' of thyroxine tablets.

1. Ask laboratory to measure thyroid function
2. If free T4 level if less than 160 pmol/l, no specific treatment
required.
3. If Free T4 level is greater than 160 pmol/l admit patient for 10 days

My understanding of the last discussion regarding T4 overdose was (this
could form the basis some guidelines if agreed):-

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THYROID HORMONE OVERDOSE

There is not a clinical case for measuring thyroxine levels urgently:-

· In the sort term, end-organ response and serum T4 levels are not
correlated. It is only in the long term, after at least 3 weeks
continuous dosage that symptoms and serum levels correlate.

· In patients, who do not normally take T4, even taking a lot of
thyroxine tablets on a single occasion does not make them clinically
ill, though it will raise serum T4 levels.

· In patients, who take T4 regularly, a change in dose takes a week or
two to become clinically effective.

Therefore, even if analysed urgently, T4 levels would not give
information on which to base treatment, except to confirm T4 had been
taken. Treatment of a patient with thyrotoxic symptoms thought to have
taken a T4 overdose would be beta blockers to reduce the effect on end
organs and to wait for the T4 to be cleared.

FREE T4 LEVELS IN THYROXINE OVERDOSE
 
When T4 is taken orally, there is a post-absortion peak and T4 is
rapidly bound to TBG which has a large reserve of binding sites (in
euthyroid state with normal levels of TBG, TBG is no more than 25%
saturated). Therefore, serum free T4 in thyroxine overdose patients does
not rise as much as may be expected. Levels fall with a normal half-life
of about 6 days.

TSH LEVELS IN THYROXINE OVERDOSE

TSH is not immediately suppressed but falls after a day or so, and as
the free T4 falls, the TSH returns to where it started from. Therefore,
'front-line TSH' in thyroxine overdose is inappropriate as it will give
a misleading answer for a day or two.


OTHER URGENT THYROID REQUESTS

Urgent thyroid requests are very rarely required and never overnight. In
the case of a long bank holiday the only clinically justifiable
situation where TFTs may be helpful would be in a case of thyroid storm
in which the clinical diagnosis was in doubt.
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Any comments?

Tony Everitt
Basildon Hospital


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