Many of our members ask WHY we should not take any thyroid hormone replacement for 24 hours before we have our blood drawn to test our thyroid function, This is because if we take our thyroid hormone replacement the morning we have our blood test done, the results could be skewed. Here is information from Professor Anthony Weetman Head of Medical Schools (UK) and an expert witness giving evidence at a General Medical Council Hearing.
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http://www.tpa-uk.org.uk/skinner_hearing_transcripts.php

In the Official Transcript Day 4 page 13.

Q: What about the laboratories that do offer the T3 and the T4? We are looking at people before they have received any medication, before they are on thyroxine. Is T3 and T4 in those circumstances a reasonably accurate test or not?

A: I do not know of a laboratory which routinely offers free T3, so let us stick with free T4. Free T4 assays are as reliable as TSH. Let me be clear on this, there are a number of factors which will interfere with these assays which are well recognised by endocrinologists and have to be taken into account. Provided one does that then the assays are equally reliable.

Q: What about once the patient is taking thyroxine?

A: If I can just continue on the last point. One of the reasons for not relying on the free T4 is that in the earliest stages of thyroid failure, so called sub-clinical hypothyroidism, the TSH levels go up but the free T4 levels remain normal. In that situation you have a partially damaged thyroid gland. If I go back (The witness demonstrated on the chart) If there is slight damage to the thyroid gland the TSH levels will rise in order to stimulate the gland.

Q: In order to compensate?

A: Exactly and do so in the majority of patients. So they end up maintaining free T4 which is normal at the expense of the high TSH which is stimulating the gland and that is why you can sometimes have the situation of an elevated TSH but a normal free T4.

Q: I think we do see that in some these cases. If you have an elevated TSH but a normal T4 within the reference range, is that a signal – it is not diagnostic presumably – but is that a signal of thyroid problems?

A: It is. It would certainly need follow up.

Q: I was going to come on, if we have finished that area, to dealing with using T3 andT4 as a test once the patient is on thyroxine?

A: The problem with using free T4 measurements if a patient is taking thyroxine is that the level fluctuate after taking thyroxine treatment. Therefore, within the few hours after ingestion there can be a ten or fifteen per cent level difference in level compared to twelve to twenty four hours after ingestion. The second problem which is frequently encountered by endocrinologists is that the patients may not adhere to their treatment very strictly and may remember to take a tablet before a blood test which will give them normal T4 levels and might have omitted their tablets over the preceding weeks. Because of the sensitivity of the pituitary that can be identified by raised TSH but a normal free T4. Therefore, TSH, because it is measuring a response of the body, in this case the pituitary gland, it is by far the best measure of the nature and degree of thyroid hormone replacement.
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Please note how skilfully Prof Weetman skirts around the issue of the T3 …..

Although he does not say it in such words, he is insinuating that the correct level for measuring FT4 is 12 to 24 hours after ingestion. And to be on the safe side (and not risk our doctors to reduce our medication) – on this forum it is generally advised to lay off the pills for at least 24 hours.