Another source:
Scand J Clin Lab Invest 1992;52:435-45 K.Liewendahl
Thyroid function tests: performance and limitations of current
methodologies.
gives as minimum significant alterations:
T4 17-23 nmol/L
FT4 6 pmol/L
T3 0.5 nmol/L
TSH 0.7 mU/L
Best regards,
Raymond Wulkan
Rotterdam
Date sent: Tue, 18 Jan 2000 22:24:13 +0000
Subject: Re: Half-life of T4 and patient compliance
From: Aubrey Blumsohn <[log in to unmask]>
To: [log in to unmask]
Send reply to: Aubrey Blumsohn <[log in to unmask]>
If I remember correctly the half life of TOTAL T4 is about 3 days and of T3
about a week. I
guess that the post-dose change in Free T4 may be somewhat more pronounced
than total T4.
Post dose changes in total T4 are fairly small but it has been suggested
that it might be worth standardising the time of sampling in relation to dose.
The long half life explains why there is minimal circadian variation in FT4
or T3 but a large circadian variation in TSH (ignored by us, but is in fact
fairly large - if the order of 2mIU/L).
Clearly the relationship between FT4 and TSH would be expected to be
different in patients on thyroxine versus the general population (since the
ratio of T4 to T3 is very different). Also bear in mind that it can take
months for TSH to suppress appropriately even given good compliance.
Aubrey Blumsohn
>Does anyone have any data or references to the half life of T4, when
>given to treat hypothyroidism?
>
>If this is hours rather than days, could the length of time from the
>patient taking the last dose to blood sampling significantly alter the
>serum thyroid hormone results?
>
>In these patients we do sometimes see a relatively high FT4 compared to
>the TSH, e.g. a raised TSH with FT4 at the upper limit of normal or
>there abouts. Our assays have been in control at the time. I know there
>are other possible explanations, but am I right that this could be due
>to poor patient compliance (raising the TSH), with the conscience
>stricken patient taking too many T4 tablets to compensate (pushing up
>the FT4) just before a clinic appointment?
>
>Has anyone carried out or come across a study looking at this?
>
>I hope to gain from your many years of collective experience of thyroid
>function.
>
>Best wishes
>
>Steve
>
>Dr S J Frost
>Clinical Biochemistry Department
>The Princess Royal Hospital
>Haywards Heath
>West Sussex
>England
>[log in to unmask]
>
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