Usually, our doctors tell us that fasting is not necessary for thyroid blood tests, nor does it matter when tests are done -- particularly TSH tests. Only those patients taking an external source of T3 -- such as Cytomel, Armour Thyroid, Nature-throid, or a time-released T3 medication -- need to be aware of when they've taken their medication and the timing of testing, as T3 is active only for hours in the body, while T4, both synthetic and natural, has a much longer period of activity. Given that TSH tests reflect the impact of several weeks of medication, it's hard to argue that time of day or fasting would have an impact on the testing and results. Interestingly, however, research by Scobbo et. al. in 2004 showed that TSH tests declined in 97 of 100 of the people studied -- by an average of 26.39% -- when compared to early morning, fasting, TSH test results. This resulted in as many as 6% of patients being reclassified from the diagnosis of subclinical hypothyroidism to "normal."
Other information actually supports the idea that patients should not take any thyroid medication the day of testing, until after tests are completed. The Thyroid Manager online textbook mentions that
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.