Re: Do you fast before labwork?
Originally Posted by sissy1
Just curious if your Doctor told you to skip your thyroid meds before lab work? It seems that not taking them would give a incorrect reading and would not show your levels as they would be on the meds. As far as not eating before thyroid test, fasting isn't necessary unless you do a cholestrerol test with it as I often do.
The online thyroid textbook at [url]www.thyroidmanager.org[/url] provides this information in regards to the timing of blood tests in relation to taking meds:
Serum T4 concentrations peak 2 to 4 hours after an oral dose and remain above normal for approximately 6 hours in patients receiving daily replacement therapy. The long half-life of thyroxine of about 7 days allows treatment with a singly daily tablet... |
Liothyronine. After oral administration of L-triiodothyronine sodium (which is more readily absorbed than T4) peak levels of serum T3 are observed within 2 to 4 hours...The half-life of T3 is approximately one day.
Dessicated thyroid (Armour--Because of the relatively high ratio of T3 to T4 in desiccated thyroid, patients receiving an amount of this medication adequate to normalize serum TSH generally have serum T4 concentrations in the lower half of the normal range. Serum T3 concentrations will vary in such patients, depending on the interval between ingestion of the medication and the time of blood sampling.
The time course of the absorption of T3 is similar whether it is contained in thyroglobulin or free in the tablet, with peak levels approximately 2 to 4 hours after oral administration
What this all means in normal language is that a within a few hours after taking your thyroid meds--regardless of what you take--your T3 and/or T4 will be abnormally high for up to 6 hours. Some doctors know to compensate for this when they look at the labs but most do not. How many times has your doctor asked you what time you took your meds on the day of your labwork? Never? I thought so.
Keep in mind any time you do a lipid profile (cholesterol/triglycerides) or CMP or any type of glucose testing you SHOULD fast.
I am not a doctor, nor have I ever played one on TV...
Last edited by Meep; 01-17-2004 at 07:50 PM.