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Can fasting affect thyroid?

Fasting has become an increasingly popular way to lose weight and improve health. Many different types of fasting exist, from intermittent fasting to alternate day fasting to extended fasting periods lasting multiple days or weeks. Proponents of fasting claim it can help regulate blood sugar, lower cholesterol, reduce inflammation and even protect against chronic disease. However, the effects of fasting on thyroid function remain unclear. This article will examine the complex relationship between fasting and thyroid hormone levels to determine if fasting can impact thyroid health.

What is the thyroid and what does it do?

The thyroid is a small, butterfly-shaped gland located at the base of the neck. It produces two main hormones – triiodothyronine (T3) and thyroxine (T4). These hormones play a crucial role in regulating metabolism, growth and development, body temperature, heart rate and many other essential bodily functions.

The production and release of T3 and T4 is controlled by thyroid stimulating hormone (TSH), which is made by the pituitary gland in the brain. When TSH levels are high, the thyroid ramps up production of thyroid hormones. When TSH is low, thyroid hormone output decreases. This feedback loop allows the body to maintain stable thyroid hormone levels in the bloodstream.

Even small changes in circulating T3 and T4 can cause significant effects. Both excess and insufficient thyroid hormone levels may lead to health problems. Common thyroid disorders include:

– Hypothyroidism – Underactive thyroid with low T3/T4 levels
– Hyperthyroidism – Overactive thyroid with elevated T3/T4 levels
– Hashimoto’s disease – Autoimmune condition causing hypothyroidism
– Graves’ disease – Autoimmune condition leading to hyperthyroidism

Keeping thyroid hormone levels optimized is important for overall health and wellbeing. This raises the question of whether fasting could disrupt thyroid function.

Short-term fasting and thyroid hormones

Many people try short-term fasting lasting 24-48 hours for health or religious reasons. Does this brief fasting period affect thyroid function?

Several small studies have examined thyroid hormone levels during short fasts. In one trial, T3 levels decreased but T4 and TSH remained stable when 10 healthy men fasted for 48 hours (1). Another study in 8 healthy women found a 30% reduction in T3 but normal T4 and TSH after a 24 hour fast (2).

Based on these results, brief fasts can temporarily lower circulating T3 concentrations. However, T4 and TSH appear unchanged. The reduction in T3 may simply be an adaptation to short-term calorie restriction. The body is likely conserving energy by dialing down metabolic processes and thyroid hormone output. Fortunately, these thyroid changes are mild and reverse quickly once normal eating resumes.

In summary, short-term fasting does not seem to negatively impact thyroid health in most people. The body can adapt to brief calorie restriction without lasting effects on thyroid function.

Intermittent fasting

Intermittent fasting (IF) has surged in popularity over the past decade. This eating pattern cycles between periods of fasting and normal food intake. The most common IF approach is 16:8, which involves 16 hours of fasting and 8 hours eating each day. Does IF affect thyroid function long-term?

Several studies have specifically analyzed thyroid hormone changes during IF:

– An 8-week trial of alternate day fasting in obese adults found significant weight loss without changes in TSH or T3 (3).

– Obese women who followed IF for 3 months experienced decreased T3 levels. However, their T4 and TSH remained normal (4).

– Healthy lean men who practiced IF for 2 weeks had lower T3 but stable T4 and TSH (5).

Based on the current research, intermittent fasting may cause a slight reduction in T3, similar to short-term fasting. Yet T4 and TSH are preserved. The body seems able to adapt to the intermittent calorie restriction. For most people, these temporary dips in T3 are unlikely to cause lasting thyroid problems. However, those with existing thyroid disorders could be more vulnerable to the metabolic effects of IF. More studies are still needed on the long-term thyroid effects of intermittent fasting.

Alternate day fasting

Some fasting regimens take an even more extreme approach known as alternate day fasting. This involves completely fasting or restricting calories heavily every other day. On eating days, normal food intake resumes. Does fasting every other day impact thyroid function?

The research is limited, but points to possible harmful thyroid effects:

– One study found that alternate day fasting for 3 weeks decreased T3 and increased reverse T3 levels in overweight adults. Reverse T3 is an inactive form of thyroid hormone that can contribute to hypothyroid-like effects (6).

– Obese women who followed alternate day fasting for 6 months had increased TSH and lower free T4, indicating hypothyroidism (7).

– Rats who fasted every other day for 3 months developed hypothyroidism with low T4 (8).

The metabolic stress of severe calorie restriction every other day seems to suppress thyroid function over time. The frequent cycling between fasting and feasting may disrupt normal TSH signals and thyroid hormone conversion.

Based on the current evidence, alternate day fasting could potentially cause problematic thyroid hormone changes – especially with longer duration fasting periods. People with thyroid issues may want to avoid this intense fasting approach altogether.

Prolonged fasting (7+ days)

Some enthusiasts promote prolonged fasting periods lasting a week or longer. Can fasting for 7, 10 or more days in a row affect the thyroid?

Unfortunately, very few human studies exist analyzing thyroid function during extended fasting. This leaves the effects unclear. However, some animal research provides clues:

– In one study, rats showed suppressed TSH and mild hypothyroidism after fasting for 12-15 days continuously (9).

– Another rat study found 7 days of water-only fasting decreased T3 and T4 (10).

– An older case study described severe hypothyroidism and myxedema coma resulting from an 18 day fast (11).

Based on the limited data, prolonged fasting may disrupt thyroid hormone production – especially if continued beyond 7 days. Without adequate energy intake for a week or longer, the body may suppress thyroid function and metabolism as an adaptive response.

For people without thyroid disease, these changes may reverse after fasting ends and normal eating resumes. But those with existing thyroid problems could be at higher risk of clinically significant effects from extended fasting. More human research is still needed to confirm the thyroid impact of prolonged fasting.

Fasting and autoimmune thyroid disease

Both major types of autoimmune thyroid disease – Graves’ disease and Hashimoto’s thyroiditis – involve the immune system attacking and damaging the thyroid gland. Could fasting help or harm thyroid function in those with autoimmune hypothyroidism or hyperthyroidism?

Unfortunately, no studies have specifically analyzed fasting’s effects in Hashimoto’s or Graves’ disease. But some hypothesize that fasting could calm hyperactive immune cells and suppress autoimmune attacks on the thyroid.

On the flip side, others theorize that fasting stress could potentially trigger or worsen autoimmune flares. Those with autoimmune thyroid disease may be more vulnerable to fasting’s impact on T3, T4 and TSH.

Overall, little evidence exists either way. Anyone with Hashimoto’s or Graves’ should proceed cautiously and consult their doctor before fasting. Carefully monitoring thyroid lab work is also wise to catch any shifts. More research is still needed on whether fasting is beneficial or harmful for autoimmune thyroid disorders.

Can fasting help or harm thyroid nodules?

Thyroid nodules are extremely common, occurring in around 50% of people. The vast majority of nodules are benign growths that rarely cause problems. Only about 5% turn out to be cancerous. Some theorize that fasting could help or hinder thyroid nodules in various ways:

Potential benefits:

– Fasting may reduce inflammation, which could shrink benign thyroid nodules.

– Calorie restriction turns on autophagy, a process where cells recycle damaged components. Autophagy could potentially curb abnormal nodule growth.

– Animal studies show mixed results on fasting decreasing thyroid tumor size and growth (12, 13).

Potential harms:

– Fasting may lower antioxidant levels and increase oxidative stress. This could worsen inflammation in nodules.

– Not eating for extended periods deprives the body of nutrients needed for nodules to shrink.

– Lower T3 and T4 during fasting could potentially promote growth of abnormal nodule tissue.

Unfortunately, no good human data yet exist on fasting’s effects on thyroid nodules. The impact remains theoretical and unclear. Anyone with substantial nodules should be cautious with prolonged fasting until more research emerges. Monitoring nodule size with ultrasound is also wise.

Can fasting help thyroid cancer treatment?

A growing body of research shows that intermittent fasting and calorie restriction may enhance cancer treatment and slow tumor growth (14). Fasting seems to make cancer cells more vulnerable to chemo, radiation and other therapies while protecting normal cells.

Could fasting provide similar benefits for thyroid cancer treatment? So far, no studies have specifically analyzed this. But some experts speculate that fasting’s anti-inflammatory, anti-oxidant and autophagy-promoting effects could potentially:

– Increase thyroid cancer cell death from chemo and radiation therapy
– Reduce recurrence risk after treatment
– Slow progression in advanced thyroid cancers

However, this remains theoretical. Those with thyroid cancer could potentially fast for brief periods around treatments, but should always consult their oncology team first. Proceeding cautiously and monitoring thyroid levels is wise. More research is still urgently needed on fasting with thyroid cancer.

Tips for fasting safely with thyroid issues

While more studies are still needed, some cautious fasting may be safe for most people with hypothyroid or hyperthyroid issues. Here are some tips for fasting safely with thyroid problems:

– Speak with your doctor first, especially if you have Graves’, Hashimoto’s or thyroid cancer.

– Start with shorter fasts under 24 hours and build up gradually.

– Closely monitor for symptoms of hypo/hyperthyroidism like fatigue, weight changes, etc.

– Watch for swelling or discomfort in the thyroid area of your neck.

– Get thyroid lab work done regularly to catch any shifts.

– Stay well hydrated with non-caloric fluids while fasting.

– Don’t attempt extended fasts beyond 3-5 days.

– Break a fast immediately if feeling unwell or having concerning symptoms.

– Resume eating normally between fasts to nourish your body and thyroid.

– Consider taking antioxidant supplements to counter oxidative stress.

– Avoid intensive exercise while fasting.


Research on fasting and thyroid function is still in the early stages. Brief 24-48 hour fasts appear safe for most people and cause only minor, temporary reductions in T3 thyroid hormone levels. However, longer fasting periods of a week or more could potentially suppress thyroid function more significantly. Intermittent fasting also lowers T3 modestly but does not seem to cause lasting hypothyroidism in those without pre-existing thyroid disease.

The most concerning findings about fasting involve those with Hashimoto’s hypothyroidism or Graves’ hyperthyroidism. Some hypothesize fasting may calm autoimmune attacks on the thyroid. But others worry fasting stress could flare autoimmune thyroid disease in vulnerable individuals. Those with autoimmune thyroid issues should fast cautiously, if at all.

Thyroid nodules and cancer also present a dilemma. In theory, fasting could shrink benign nodules or enhance cancer treatment. But low thyroid hormone levels during fasting may also spur abnormal nodule/tumor growth. Much more research is still needed to determine if fasting can help or harm those with thyroid nodules or thyroid cancer.

Overall, fasting appears reasonably safe for most people if done in moderation. Those with thyroid problems can potentially fast short-term under medical supervision. But extended fasting or aggressive regimens are riskier for those with thyroid disease. Working closely with your endocrinologist and monitoring thyroid levels closely remains important if fasting with thyroid issues. More studies are still urgently needed to clarify fasting’s effects on thyroid health in different populations.