Thyroid UK
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What’s the reason for stopping Levo before blood test?

Why is it advised to stop taking Levo 24 hours before blood tests? Won’t that give an inaccurate result? If stopping causes an increase/ decrease in TSH and your dosage is adjusted accordingly wouldn’t that cause more harm than good? Surely if you continue taking Levo the blood test would show if you’re dosage is accurate.

Sorry I’m sure that’s probably a stupid question but I’m due a blood test Friday and I want the results to be accurate. I’m also having the medicheck thyroid ultra vit test and I don’t want to waste money if I’m not doing the test correctly.

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What you want is blood tests drawn at a similar time of day a similar length of time after the last dose of levothyroxine. That way you can compare blood results over time and monitor the effect of dose changes. TSH is at its highest in the morning and starts to fall after food. Levels of levothyroxine peak in the body 4 to 6 hours after taking a dose which is reflected in the blood results. Liothyronine has a shorter half life so peaks roughly 2 hours after taking and is normally out of the system after 12 hours.

Therefore I choose to get blood taken first thing and delay food and medication until after the blood draw, so if blood is taken at 8am or 10am the conditions are similar and the results can be compared. A colleague always has blood taken around 14.00 having taken her levothyroxine at 5am so she too can compare her results.

The variation in TSH during the day varies according to individuals with some people having a significant difference. If you feel stable and well then replication of previous conditions for a blood test is reasonable, but if you feel undermedicated, trying to record your highest TSH may be helpful.

Hope this helps.

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Cjrsquared, thank you for the comprehensive response. I had my first blood test in the afternoon 6 weeks after diagnosis and the last one was in the morning. The first test was TSH 2.9 the second was 3.73. I wrongly assumed that the increase on the second test showed I was under medicated when it’s entirely possible it was because the tests were done at different times of the day. I took levo before both tests but will hold off this time. I think it will be a while before I can make a comparative analysis of any of my results. 😜

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Taking your dose before the test won't affect TSH, because it doesn't move that fast. But, it will give you a high FT4 level, rather than the normally circulating level of T4 - all you'll be testing is the dose you've just taken.

Even a TSH of 2.9 shows that you are under-medicated - over 3 shows that you are still hypo - Because once you are on thyroid hormone replacement, your TSH should be one or under. As doctors only tend to look at the TSH, it's best to have it as high as possible, if you are hypo and want an increase in dose. :)

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Hi GreyGoose I was hoping you’d comment, thank you! I don’t understand if it doesn’t affect TSH and that’s all they test for how will not taking it give me a higher TSH? Sorry I’m still under medicated so unfortunately I’m still DUMB! Lol 😂

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Rosserk, labs will sometimes unexpectedly test FT4 if they think that your TSH is on the low side... so if your FT4 came in too high they might advise the doc that you're overmedicated... resulting in dose being reduced So best not to take any chances.

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Thank you RedApple, I’m new to the thyroid maze and I’m afraid I’m not behaving very well! I seem to have to ask the same question in several ways in order for it to register. Kindest regards 😆

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No problem at all, we've all been there :D

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Hopefully I won’t be there much longer! 😂

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