24-Hour Fasting Raises Reverse T3 and Lowers Active Thyroid Hormone: What the Research Shows

24-Hour Fasting Raises Reverse T3 and Lowers Active Thyroid Hormone: What the Research Shows

Medical disclaimer: This article summarises published research for informational purposes only. It is not medical advice and is not a substitute for guidance from a qualified health professional. Always consult your doctor before starting any fasting protocol, especially if you have an existing health condition or take medication.

Study at a Glance

What This Study Looked At

Researchers at the NIH's metabolic research unit wanted to know what a single day without food does to thyroid hormone levels in healthy people — and how that compares with eating more than usual. This matters for anyone doing intermittent fasting or time-restricted eating, since concerns about fasting "crashing" thyroid function are common, especially among women managing existing thyroid conditions. The team tested a 24-hour fast alongside a normal-calorie diet and several higher-calorie diets with varying protein content, all within the same group of volunteers.

Who Was Studied

Participant profile: Healthy, euthyroid (normal baseline thyroid function) adults with normal glucose regulation, studied at the NIH's Phoenix, Arizona research unit.

How the fasting protocol worked in this study: Each participant completed all dietary conditions in turn, in a randomized crossover design, with each 24-hour period conducted inside a whole-room indirect calorimeter so the research team could also measure energy expenditure alongside blood hormone levels.

What the Researchers Found

Thyroid Hormone Conversion

The clearest finding was a shift in how the body converts thyroid hormone during the fasting day compared with normal eating:

• The body's active thyroid hormone (T3) dropped modestly after just one day of fasting, while reverse T3 — an inactive form that essentially acts as a "brake" on metabolism — rose noticeably over the same period.
• This pattern is consistent with the body's well-documented tendency to reduce the conversion of T4 into active T3 during energy shortfall, redirecting more of it into the inactive rT3 form instead.

What Did Not Change

• TSH (thyroid-stimulating hormone) and total T4 were also measured across conditions; the published abstract did not indicate the dramatic shifts seen with FT3 and rT3, consistent with the broader literature showing TSH tends to stay relatively stable during short fasts of this length.
• Participants remained classified as euthyroid throughout the study — a single 24-hour fast did not push healthy volunteers into a hypothyroid state.

What the Researchers Concluded

The authors concluded that even a brief, 24-hour fast is enough to measurably shift thyroid hormone metabolism toward reduced active hormone and increased reverse T3, a pattern that reflects the body conserving energy in response to a short-term calorie shortfall — separate from, and in addition to, the overfeeding-related findings the same study reported for the higher-calorie diets.

What This Means If You Fast

• A single fasting day is not damaging your thyroid. The FT3/rT3 shift documented here reflects a normal, reversible metabolic adaptation to short-term energy shortfall in healthy people — not evidence of thyroid injury.
• Frequent, very long fasts are a different question. This study only tested a single 24-hour window; it doesn't tell us what happens with repeated multi-day fasting, which is where women with existing thyroid conditions are typically advised to be more cautious.
• If you have a diagnosed thyroid condition, loop in your doctor. Because rT3 rises even in healthy volunteers after just one day of fasting, anyone already managing hypothyroidism or Hashimoto's has good reason to discuss fasting length and frequency with their provider rather than guessing.
• Energy dips during a fast may partly reflect this hormone shift. A modest, temporary drop in active thyroid hormone is one plausible contributor to the low-energy feeling some people notice during longer fasts, alongside the more commonly cited factors like hydration and electrolytes.
• This was healthy volunteers, not people with thyroid disease. The results describe how a normally functioning thyroid responds to fasting — they can't be assumed to apply the same way to someone whose thyroid is already underperforming.

Study Limitations

• Full-text access was restricted at the time this article was written, so some secondary numeric results (exact TSH/T4 changes, statistical significance values) could not be independently verified and are described qualitatively rather than with specific figures.
• Participants were healthy volunteers with normal thyroid function — the findings may not generalize to people with hypothyroidism, Hashimoto's, or other thyroid disorders.
• The fasting window tested was a single 24-hour period; the study does not address the effects of repeated or longer fasting protocols.
• Conducted at a single NIH research unit using a controlled inpatient/metabolic-chamber setting, which may not reflect real-world fasting conditions.

Source

Basolo A, Begaye B, Hollstein T, Vinales KL, Walter M, Santini F, Krakoff J, Piaggi P. (2019). Effects of Short-Term Fasting and Different Overfeeding Diets on Thyroid Hormones in Healthy Humans. Thyroid, 29(9), 1209–1219. PMID: 31298652

Frequently Asked Questions

Does intermittent fasting lower thyroid hormone?

A single 24-hour fast in this study modestly lowered free T3 (about 6%) and raised reverse T3 (about 16%) in healthy adults — a temporary metabolic adjustment, not a sign of thyroid damage.

What is reverse T3 and why does it rise during fasting?

Reverse T3 is an inactive form of thyroid hormone that the body produces instead of active T3 when energy intake drops. Its rise during fasting reflects the body conserving energy rather than any thyroid dysfunction.

Is it safe to fast if I have a thyroid condition?

This study only looked at healthy volunteers with normal thyroid function, so it doesn't directly answer that question. People with hypothyroidism or Hashimoto's should discuss fasting length and frequency with their doctor before starting.

Does TSH change during a 24-hour fast?

According to this study, TSH did not show the same dramatic shift seen with FT3 and reverse T3, which is consistent with other research showing TSH tends to stay relatively stable over short fasting periods.

How long does it take for thyroid hormones to return to normal after fasting?

This study did not report a specific recovery timeline, but the broader fasting literature indicates that T3 and reverse T3 levels typically normalize once regular eating resumes.

Related Research and Articles

• Intermittent Fasting and Thyroid Health in Women
• Fasting and Hypothyroidism: A Practical Guide for Women
• Time-Restricted Eating and Thyroid Function in Metabolic Syndrome: What the Research Shows
• Intermittent Fasting and Energy Levels in Women
• Growth Hormone and Fasting: What the Research Shows

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