Fasting is recommended before several blood tests and should be defined as no food or beverages except water for at least 12 hours (Kackov 2013). Although fasting is not “required” for TSH testing, it is ideal. TSH levels drop significantly following food intake, which may confound the diagnosis and monitoring of subclinical hypothyroidism. Free T4 levels are likely unaffected by fasting. In one study of 57 ambulatory subjects, fasting TSH was significantly higher than postprandial TSH, leading researchers to reclassify these individuals as having subclinical hypothyroidism (Nair 2014).
A prospective study of 52 subjects with no known thyroid issues also found that TSH decreased significantly following a meal, possibly due to a rise in somatostatin, which suppresses TSH. However, researchers also observed that when two fasting samples were obtained, the sample drawn at 10 am was significantly lower than that drawn at 8 am. An 8 am blood draw may be ideal for assessing TSH accurately. Levels of free T4 did not vary according to fasting state but did differ when different assay methods were utilized (Mahadevan 2017).
To optimize test results, it would be prudent to recommend fasting for the following tests as well (Simundic 2014, Kackov 2013):
Some nutrition supplements can influence specific blood test results.
High-dose biotin can affect TSH results and should be avoided before testing. A dose of 5 mg or more (Li 2020), or 10,000 times the RDA of 30-70 ug per day, can interfere with thyroid test results, including TSH, free T3, and free T4 (Ardabilygazir 2018).
Other restrictions should be reviewed with a healthcare professional prior to testing and should be included in the patient instructions from the laboratory.
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Ardabilygazir, Arash et al. “Effect of High-dose Biotin on Thyroid Function Tests: Case Report and Literature Review.” Cureus vol. 10,6 e2845. 20 Jun. 2018, doi:10.7759/cureus.2845
Kackov, Sanja et al. “Are patients well informed about the fasting requirements for laboratory blood testing?.” Biochemia medica vol. 23,3 (2013): 326-31. doi:10.11613/bm.2013.040
Li, Danni et al. “AACC Guidance Document on Biotin Interference in Laboratory Tests.” The journal of applied laboratory medicine vol. 5,3 (2020): 575-587. doi:10.1093/jalm/jfz010
Mahadevan, Shriraam et al. “Does Time of Sampling or Food Intake Alter Thyroid Function Test?.” Indian journal of endocrinology and metabolism vol. 21,3 (2017): 369-372. doi:10.4103/ijem.IJEM_15_17
Nair, Rakesh et al. “Does fasting or postprandial state affect thyroid function testing?.” Indian journal of endocrinology and metabolism vol. 18,5 (2014): 705-7. doi:10.4103/2230-8210.139237
Simundic, A M et al. “Standardization of collection requirements for fasting samples: for the Working Group on Preanalytical Phase (WG-PA) of the European Federation of Clinical Chemistry and Laboratory Medicine (EFLM).” Clinica chimica acta; international journal of clinical chemistry vol. 432 (2014): 33-7. doi:10.1016/j.cca.2013.11.008