Reader Adam Kadela has begun intermittent fasting and wonders how it might affect his hypothyroidism:
I have a question pertaining to the section at the end of the book covering extended fasts. I regularly practice the 16-8 fast/feast protocol (breakfast at noon, last meal before eight), and plan to throw in a 36 hour fast once a month per your book. However, I am hypothyroid (hashimoto’s) and take synthetic T4 and T3 (unithroid and cytomel), so I’m wondering if an extended fast could affect my thyroid function negatively.
This is a great question. I think the daily 16-hour fast should be therapeutic for hypothyroidism, but I’m not sure about the 36-hour fast.
In today’s post I want to talk about why daily intermittent fasting may be therapeutic for Hashimoto’s, which is an autoimmune hypothyroidism.
Food Sets The Circadian Clock
The circadian clock is strongly influenced by diet: indeed, food intake dominates light in setting the circadian clock. If you regularly eat at night and fast during the day, the body will start treating night as day and day as night. 
(Alcohol consumption at night will also tend to reset the clock, which may explain why college students are often night owls!)
This suggests that controlling the timing of food consumption can help to maintain circadian rhythms.
The Circadian Clock and Hypothyroidism
The thyroid follows circadian rhythms. There is a circadian pattern to TSH levels: high at night, low during the day.
The thyroid’s circadian pattern is diminished in autoimmune hypothyroidism. In a study of hypothyroid children, the night-time surge of TSH averaged 22%, compared to 124% in normal children. Only one of 13 hypothyroid children had a night-time TSH surge in the normal range. 
The study authors concluded:
We suggest that the nocturnal surge of TSH is important for maintenance of thyroid function and conclude that the nocturnal TSH surge is a much more sensitive test than the TSH response to TRH for the diagnosis of central hypothyroidism. 
Shift Work and Hypothyroidism
If circadian rhythms are important for thyroid function, we would expect shift workers to have high rates of hypothyroidism. Shift workers sleep during the day and eat at night, which disrupts circadian rhythms.
It turns out that shift work doubles the risk of autoimmune hypothyroidism:
Stress induces autoimmune disorders by affecting the immune response modulation. Recent studies have shown that shift work stress may enhance the onset of the autoimmune Graves hyperthyroidism. On the other hand, the possible association between occupational stress and autoimmune hypothyroidism has not yet been investigated…. Subclinical autoimmune hypothyroidism was diagnosed in 7.7 percent shift workers and in 3.8 percent day-time workers with a statistically significant difference: Odds Ratio (OR) 2.12, 95 percent Confidence Interval (CI) 1.05 to 4.29; p=0.03…. Our data show a significant association between shift work and autoimmune hypothyroidism. This finding may have implications in the health surveillance programs. 
Shift Work Affected Adam Too
In a follow up email, Adam told me that night shift work may have helped cause his hypothyroidism:
[T]he paper about thyroid and fasting … is particularly interesting to me due to my experience with night shift work for 10 months last year. My circadian rhythm was all out of whack due to experimenting with different sleep schedules and trying to workout around midnight before going into work at two a.m. I also played around with different diet strategies (grazing method w/ small meals, warrior diet, and ultimately settling on the 16-8, which is by far superior imo). My thyroid, along with other hormones, did not enjoy these trials.
Intermittent Fasting May Be Therapeutic
Since the circadian rhythm is affected by both food and light exposure, lifestyle practices can enhance natural circadian rhythms. These practices should optimize the circadian cycle:
- Light entrainment: Get daytime sun exposure, and sleep in a totally darkened room.
- Daytime feeding: Eat during daylight hours, so that food rhythms and light rhythms are in synch.
- Intermittent fasting: Concentrate food intake during an 8-hour window during daylight hours, preferably the afternoon. A 16-hour fast leading to lower blood sugar and insulin levels, and the more intense hormonal response to food that results from concentration of daily calories into a short 8-hour time window, will accentuate the diurnal rhythm.
- Adequate carb intake: Eat at least 400 “safe starch” carbohydrate calories daily during the afternoon feeding window. Relative to a very low-carb diet, this will increase daytime insulin release and, by increasing insulin sensitivity, may reduce fasting insulin levels. It will thus enhance diurnal insulin rhythm.
Adam tells me that intermittent fasting seems to be improving his hypothyroidism:
I think you’re correct in that I’ve experienced some curative effects. However, with the improved nutrient absorption and gut health from healthier eating and fasting, I think I fluctuate a lot b/w slightly hypo, normal, and hyper, since my medication is constant. I’m still in the process of finding a balance, but it’s a bigger improvement than my past state.
Many doctors mistakenly assume that little can be done to cure autoimmune disorders. In fact, however, autoimmune conditions commonly disappear once the chronic infections, food toxins, or poor health practices that cause them are eliminated.
Circadian rhythms have powerful influences on many biological processes, and disrupted circadian rhythms are a common feature of disease. Without clinical trials it’s impossible to be sure, but efforts to enhance circadian rhythms may be therapeutic for diseases such as hypothyroidism.
Intermittent fasting, daytime light exposure, excluding light from the bedroom, night fasting and daytime feeding are simple practices. But they may be underappreciated keys to good health.
 Fuller PM et al. Differential rescue of light- and food-entrainable circadian rhythms. Science. 2008 May 23;320(5879):1074-7. http://pmid.us/18497298.
 Rose SR et al. Hypothyroidism and deficiency of the nocturnal thyrotropin surge in children with hypothalamic-pituitary disorders. J Clin Endocrinol Metab. 1990 Jun;70(6):1750-5. http://pmid.us/2112153.
 Magrini A et al. Shift work and autoimmune thyroid disorders. Int J Immunopathol Pharmacol. 2006 Oct-Dec;19(4 Suppl):31-6. http://pmid.us/17291404.