Very strict carbohydrate restriction can lower free T3 — this has been documented for decades. A classic 1976 study found that a carbohydrate-free, very-low-calorie diet lowered T3 by about 47% over 2 weeks, while diets containing at least 50 grams of carbohydrate a day showed no significant change in T3. TSH, meanwhile, tends to stay within the normal range despite these shifts.

Can someone with Hashimoto's do keto?

Hashimoto's alone doesn't rule out keto, but it calls for caution. A 2025 study randomized 40 Hashimoto's patients to either a low-carbohydrate diet or their normal diet; after 6 months, the low-carb group showed lower thyroid tissue inflammation markers and lower TPOAb/TgAb antibody levels, while the normal-diet group showed no change. Note the study used 'low-carbohydrate,' not necessarily a strict ketogenic protocol — the two aren't identical.

I take levothyroxine — what should I do before starting keto?

Talk to your doctor and get your TSH, free T3, free T4 and antibody levels (anti-TPO, anti-Tg) checked before starting, then recheck after 6-8 weeks. Weight change can also affect how your medication is absorbed, so never adjust the dose on your own.

Is fatigue and feeling cold on keto a sign my hypothyroidism is getting worse, or just normal adaptation?

The two can look very similar and it isn't safe to tell them apart on your own. 'Keto flu' fatigue, headaches and feeling cold in the first week or two is usually a temporary adaptation sign. If it lasts longer than a few weeks, or comes with hair loss, constipation or irregular periods, get bloodwork done to see whether your thyroid function has actually changed.

Health

How Does Keto Affect Thyroid Health? The Evidence on Hashimoto's and T3

9 min read · Health Guide · October 4, 2026

"I have a thyroid condition — can I do keto?" is a question that comes up constantly, mostly from women, since thyroid disorders are far more common in women than men, and there's a widespread fear that cutting carbs will throw hormones further off balance. The real picture has more layers than that fear suggests: low carbohydrate intake does have a measurable effect on thyroid hormone production, but a new study has also found an unexpectedly positive effect on the inflammation underlying Hashimoto's. Here we treat the two separately rather than mixing them together.

Very low carbohydrate intake can lower T3 — a finding that's been around for 50 years. But a 2025 study found that low carb also reduced inflammation markers in Hashimoto's. Both can be true at once, and where the balance lands depends on the person.

Carbohydrate and T3: A Half-Century-Old Finding

The thyroid gland produces T4 (thyroxine), but most of the actual work in the body is done by T3 (triiodothyronine), and the conversion of T4 to T3 is sensitive to carbohydrate intake. This isn't a new discovery: a classic study published in the Journal of Clinical Endocrinology & Metabolism in 1976 put obese participants through total fasting (7-18 days) and 800-calorie diets with varying carbohydrate content. During total fasting, serum T3 fell by an average of 53%, with a reciprocal 58% rise in reverse T3 (rT3 — an inactive metabolite). On a carbohydrate-free 800-calorie diet, T3 fell by a similar roughly 47% over 2 weeks, though this time rT3 showed no significant rise over time. By contrast, isocaloric diets containing at least 50 grams of carbohydrate a day showed no significant change in either T3 or rT3.

The conditions in that study (fasting, or an 800-calorie very-low-energy diet) are more extreme than an ad libitum keto diet eaten to satiety in everyday life. But the underlying message still holds: the body has an adaptive mechanism that dials down T3 production to conserve energy when carbohydrate (and sometimes total calorie) intake drops, and that threshold tends to sit somewhere around 50 grams of carbohydrate a day. KetoDiyetim's standard plan targets 20-25 grams of net carbs a day — below that threshold. That doesn't mean keto is harmful; it means thyroid function is worth monitoring while you do it.

TSH Usually Stays Calm — That Can Be a Trap

Here's the most confusing part: while T3 falls, TSH (thyroid-stimulating hormone) tends to stay within the normal range. A standard thyroid screen often checks only TSH, so a normal TSH gets read as "thyroid is fine" even though free T3 may have dropped. That's why anyone with a history of thyroid disease, or anyone noticing low energy, feeling cold, or hair loss on keto, gets a more reliable picture by also checking free T3 and free T4, not just TSH.

The New Finding on Hashimoto's: Inflammation May Go Down

The other side of the coin comes from a study published in mid-2025 in Medical Science Monitor. Researchers randomized 40 patients with Hashimoto's thyroiditis into two groups: one continued their normal diet, the other switched to a low-carbohydrate diet. After six months, advanced MRI imaging measured water content in thyroid tissue (an indirect marker of inflammation): the low-carb group showed a significant decrease, and the same group also saw lower thyroid peroxidase antibodies (TPOAb) and thyroglobulin antibodies (TgAb). The normal-diet group showed no significant change on any measure.

One distinction matters here: the "low-carbohydrate" diet used in that study may have been more moderate than KetoDiyetim's strict ketogenic protocol (20-25g a day) — the paper doesn't spell out an exact gram target. So this finding shouldn't be read as "keto cures Hashimoto's." The more accurate reading is that meaningfully cutting carbohydrate doesn't have to increase thyroid inflammation — in at least one study population, it reduced it instead. This doesn't contradict the T3 finding above — one is a hormone-production adaptation, the other an immune process, and both can happen in the same person at once.

What Should You Actually Do?

If you have a history of thyroid disease, talk to your doctor before starting keto and get free T3, free T4, and anti-TPO/anti-Tg antibodies checked alongside TSH. A recheck after 6-8 weeks lets you see the change in real time.
If you take levothyroxine or another thyroid medication, never adjust the dose yourself. Weight loss can change how the medication is absorbed; dose changes should always happen under medical supervision.
You don't have to keep net carbs near zero. If you have a thyroid history and notice fatigue or cold intolerance, raising net carbs to the 30-50 gram range (sometimes called "modified keto") can ease the pressure on T3 while usually still maintaining ketosis.
Learn to tell keto flu apart from worsening hypothyroidism. Fatigue or headaches in the first 1-2 weeks are usually temporary adaptation (see our electrolyte balance article); fatigue lasting weeks, or combined with hair loss, calls for bloodwork (see our article on hair loss on keto).
Know how keto can affect bloodwork more broadly: we cover that in a separate article on keto and blood test results.

Limitations

The 1976 study is small and short by today's standards, and was done in people with obesity — it may not translate directly to someone at a healthy weight doing long-term ad libitum keto. The 2025 Hashimoto's study is also a single-center trial of 40 people; it's too early to treat the result as settled without a larger, multi-site replication. Neither study supports a flat conclusion like "keto treats thyroid disease" or "keto damages the thyroid." What they show is that carbohydrate intake has a measurable, but individually variable, effect on thyroid physiology.

Sources: Spaulding et al., "Effect of caloric restriction and dietary composition on serum T3 and reverse T3 in man", Journal of Clinical Endocrinology & Metabolism (1976) · Dai et al., "Quantitative Multi-Parameter MRI Evaluation of Hashimoto's Thyroiditis Changes After Dietary Interventions", Medical Science Monitor (2025)

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