Does Intermittent Fasting Affect Thyroid?
Many women with thyroid conditions, especially hypothyroidism, are concerned about the way intermittent fasting may affect thyroid function. In some respects, there is a considerable amount of research examining the impact of fasting on thyroid function, especially when it comes to iodine consumption. Iodine is critically important for the production of thyroid hormone. Without iodine, the thyroid gland cannot make thyroid hormone.1 This can lead to hypothyroidism in adults and can have devastating consequences for fetuses and children.2,3 On the other hand, there is relatively little information regarding the role that intermittent fasting has on the thyroid gland. Therefore, we review what is known about intermittent fasting for people concerned about thyroid function.
It may be helpful to consider the effect of fasting on thyroid hormones before focusing on intermittent fasting. It is important to understand that these are two very different things. Most scientific articles include experiments in which a fasting state was created by 10 days or more without food. This is clearly not the same thing as intermittent fasting and is more closely related to starvation. Nevertheless, we know from these studies that fasting decreases the concentration of T3 thyroid hormone while thyroxine (T4) and free T4 levels stay the same or only decrease slightly. Moreover, thyroid-stimulating hormone (TSH) does not increase.4,5
What does this mean? Multi-day (near starvation) fasting does change the level of the most active thyroid hormone (T3), but T4 (the precursor to T3) and the hormone used to test for thyroid function (TSH) are remarkably unchanged.
How can this be? Multi-day fasting does affect the hypothalamus-pituitary-thyroid axis, just not on the hormones that affect thyroid function in the body. Fasting changes levels of thyroid-releasing hormone (TRH) and leptin,5 yet other values are held relatively constant over several days.
The surprisingly modest effect of near-starvation fasting on thyroid hormone levels suggests that intermittent fasting would create an almost imperceptible effect on thyroid function.
Perhaps a more relevant example is the effect of Ramadan fasting on thyroid function. Observant Muslims fast during the month of Ramadan. For one month, they only eat after the sun has set. Thus, they could be said to follow a pattern of intermittent fasting—fasting for 8 to 12 hours during the day followed by nourishment.
This intermittent fasting does cause a few changes in blood measurements such as blood glucose, HDL (good cholesterol) and LDL (bad cholesterol); however, most markers are unchanged.6 In healthy individuals, this type of fasting had no significant effect on T3, T4, or TSH levels.7 There are detectable changes in these values, but they stay within normal limits.7
Based on the best available scientific information—which is still limited—intermittent fasting is probably safe for most people with hypothyroidism. Intermittent fasting is not likely to affect thyroid hormone levels or TSH to any great degree. Moreover, people with mild to moderate hypothyroidism apparently do not need to adjust their treatments for Ramadan-style intermittent fasting, either.8 Of course, if you experience symptoms of hypo- or hyperthyroidism during intermittent fasting, restart normal meals immediately and re-evaluate. Importantly, anyone embarking on an intermittent fasting regimen should consult with a physician first. This is especially true of anyone being treating for hypothyroidism.
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- Obican SG, Jahnke GD, Soldin OP, Scialli AR. Teratology Public Affairs Committee Position Paper: Iodine Deficiency in Pregnancy. Birth defects research. Part A, Clinical and molecular teratology. 2012;94(9):677-682. doi:10.1002/bdra.23051
- Li Y, Shan Z, Teng W, et al. Abnormalities of maternal thyroid function during pregnancy affect neuropsychological development of their children at 25-30 months. Clin Endocrinol (Oxf). Jun 2010;72(6):825-829. doi:10.1111/j.1365-2265.2009.03743.x
- Purandare A, Co Ng L, Godil M, Ahnn SH, Wilson TA. Effect of hypothyroidism and its treatment on the IGF system in infants and children. J Pediatr Endocrinol Metab. Jan 2003;16(1):35-42.
- Fliers E, Kalsbeek A, Boelen A. MECHANISMS IN ENDOCRINOLOGY: Beyond the fixed setpoint of the hypothalamus–pituitary–thyroid axis. European Journal of Endocrinology. November 1, 2014 2014;171(5):R197-R208. doi:10.1530/eje-14-0285
- Boelen A, Wiersinga WM, Fliers E. Fasting-induced changes in the hypothalamus-pituitary-thyroid axis. Thyroid. Feb 2008;18(2):123-129. doi:10.1089/thy.2007.0253
- Ziaee V, Razaei M, Ahmadinejad Z, et al. The changes of metabolic profile and weight during Ramadan fasting. Singapore Med J. May 2006;47(5):409-414.
- Mansi K, Amneh M. Impact of Ramadan fasting on metabolism and on serum levels of some hormones among healthy Jordanian students. J Med Sci. 2007;7(5):755-761.
- Raza SA, Ishtiaq O, Unnikrishnan AG, et al. Thyroid diseases and Ramadan. Indian J Endocrinol Metab. Jul 2012;16(4):522-524. doi:10.4103/2230-8210.98001
Thank you for writing this, Dr. S.
Your intermittent fasting protocols are changing my life in such amazing ways. I so appreciate the work you do, and intelligence you bring to fitness.
Keep rockin’ it, girl. My condolences regarding Taz. XO