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Thyroid UK
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What affects TSH test?

Hi there,

I've having a blood test at GP's this Friday and am really hoping that they will increase my Levo from 75mcg to 100mcg as fairly convinced 75 is not enough.

Does anyone know if supplements can affect the accuracy of the test?

Test is first thing in the morning. I take Feroglobin (Iron, B6, B12, Zinc, Copper) with Vitamin C at lunch and Vit D3 + K2 and Magnesium in the evening.

Thanks for any pointers

3 Replies

Biotin (single supplement or in a B Complex) can affect results so should be left off for about 5 days. The others you mention shouldn't have any effect, but personally I would take them after the blood draw on the day of your test.

As you are looking for an increase in Levo, don't take it on the morning of the test, take it after the blood draw, fast overnight from last evening meal (you can have water, breakfast after blood draw), and have the earliest appointment of the day for blood draw, no later than 9am. Those conditions give the highest TSH which is needed when looking for an increase in dose or to avoid a reduction.

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Thanks :)


Time of day, whether or not you have eaten, sex hormones, health of pituitary and hypothalamus and, of course, levels of T4 and t3, all affect TSH levels. Biotin can make the test results invalid (TSH and free T4), so needs to be stopped about a week before test - check to see if there is biotin in anything you are taking.

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