Missing dose question..

[Dee8686]

I know the patient to patient tip is to miss a levothyroxine dose before a blood test so that enables TSH to be at its highest, but can someone please tell me why? Does that not confuse where your TSH actually is whilst taking your dose? Sorry to sound thick.. Someone in my family has questioned me after saying this is hence the post! Thanks

[SeasideSusie Remembering]

It's not leaving off Levo for 24 hours that gives the highest TSH, it's actually having blood drawn as early as possible and fasting overnight.

TSH is highest in the early hours of the morning and lowers throughout the day so an early blood draw will give the highest possible TSH. Eating lowers TSH so again fasting overnight go especially the highest TSH. If we are looking for a diagnosis or if we want to avoid a reduction in our dose of thyroid meds (or looking for an increase) then we need the highest possible TSH. As most doctors are TSH obsessed and tend to dose only by TSH, then sometimes it in our best interest to ensure we have a higher TSH when tested.

As for timing of taking last dose of Levo, if testing is done close to taking it then as T4 level peaks 2-6 hours after taking Levo then the result would show that you'd just taken your Levo and not what's normally circulating in your blood.

[Clutter]

Dee8686,

The advice is to delay taking the dose until after your blood test, not to skip taking a dose. Levothyroxine can peak in the blood stream for up to 8 hours after ingestion so taking a dose before your blood test may cause a rise in T4. You want normal circulating levels measured not the peaks caused by taking the medication.

It doesn't affect TSH at all. It will take about a week for TSH to respond to a dose. Advice to test early and fast is because TSH is highest early in the morning and drops after eating and drinking.

[Dee8686 in reply to Clutter]

Thank you both!

[cjrsquared]

The other reason for fasting and early morning blood draw is to try and get comparability between blood tests. If you are 3 hours post levothyroxine for one, then afternoon blood test for another it is more difficult to compare one against the other, especially if you have had a dose change and want to measure effect.

[Lilian15]

Here is information from Professor Anthony Weetman

Quote: 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.

[Phoenix605]

Thyroid hormones follow a circadian rhythm, heres the scientific bit if your relatives are interested, lol. There is a TSH graph on the top of page 2.

reboundhealth.com/cms/image...