Why Your “Low-Carb” Diet Could Be Harming Your Thyroid

In recent years, low-carbohydrate diets have gained immense popularity as a quick solution for weight loss, blood sugar control, and metabolic health. From the ketogenic diet to Atkins and paleo, millions swear by the benefits of cutting carbs. However, while these diets may offer short-term results, emerging research suggests they could be silently sabotaging a crucial regulator of your metabolism—your thyroid.

The thyroid gland, a small butterfly-shaped organ in your neck, produces hormones that influence nearly every cell in your body. These hormones—primarily thyroxine (T4) and triiodothyronine (T3)—control metabolism, energy production, body temperature, and even mood. When thyroid function falters, symptoms like fatigue, weight gain, hair loss, and brain fog can emerge. Alarmingly, restrictive low-carb diets may contribute to thyroid dysfunction, particularly in susceptible individuals.

The Link Between Carbohydrates and Thyroid Function

1. Carbohydrates and T3 Production

One of the most critical ways carbs influence thyroid health is through their role in converting inactive T4 into active T3. Research shows that when carbohydrate intake is too low, the body reduces T3 production as a protective mechanism. This is because T3 is highly metabolically active, and in times of perceived starvation (such as during prolonged carb restriction), the body downregulates metabolism to conserve energy.

A study published in The Journal of Clinical Endocrinology & Metabolism found that individuals on very low-carb diets had significantly lower T3 levels compared to those consuming moderate carbohydrates. This “low T3 syndrome” mimics hypothyroidism, even if standard thyroid tests (TSH and T4) appear normal.

2. Glucose Deprivation and Stress Hormones

Carbohydrates are the body’s preferred energy source, particularly for the brain and red blood cells. When carb intake is insufficient, the liver produces glucose through gluconeogenesis, breaking down proteins (muscle) and fats. However, this process is stressful for the body and triggers the release of cortisol, the primary stress hormone.

Chronic cortisol elevation suppresses thyroid-stimulating hormone (TSH) and impairs the conversion of T4 to T3. Over time, this can lead to a sluggish thyroid, even if you’re not technically “hypothyroid” by lab standards.

3. Leptin and Thyroid Regulation

Leptin, the “satiety hormone,” plays a key role in thyroid function. Produced by fat cells, leptin signals the brain to regulate metabolism and energy expenditure. Low-carb diets, especially when combined with calorie restriction, can lead to leptin resistance or low leptin levels.

When leptin drops, the hypothalamus (the brain’s control center for hormones) reduces TSH production, slowing thyroid function. This is why many people on long-term keto or low-carb diets experience stalled weight loss, cold intolerance, and fatigue—classic signs of a sluggish thyroid.

Who Is Most at Risk?

Not everyone on a low-carb diet will develop thyroid issues, but certain individuals are more vulnerable:

• Women, especially perimenopausal or postpartum women (hormonal fluctuations make them more susceptible to thyroid dysfunction).
• Those with a family history of thyroid disorders (Hashimoto’s, Graves’ disease).
• People with existing metabolic issues (insulin resistance, PCOS, or adrenal fatigue).
• Endurance athletes or highly active individuals (low-carb diets can exacerbate “relative energy deficiency in sport” or RED-S).

How to Support Thyroid Health on a Lower-Carb Diet

If you thrive on a low-carb approach but want to protect your thyroid, consider these strategies:

1. Cycle Carbohydrates

Instead of perpetual carb restriction, try carb cycling—eating higher carbs on active days and lower carbs on rest days. This prevents long-term metabolic adaptation.

2. Prioritize Nutrient-Dense Carbs

When you do eat carbs, choose fiber-rich, nutrient-dense sources like sweet potatoes, squash, berries, and quinoa over refined sugars.

3. Monitor Thyroid Markers

Beyond standard TSH tests, check free T3, reverse T3, and thyroid antibodies to catch subtle dysfunction early.

4. Support Adrenal Health

Since stress hormones impact thyroid function, prioritize sleep, manage stress, and avoid excessive fasting or over-exercising.

5. Ensure Adequate Protein and Fats

Thyroid hormones are made from tyrosine (an amino acid) and iodine. Include quality protein (eggs, fish, grass-fed meat) and healthy fats (avocados, olive oil, nuts) to support hormone production.

The Bottom Line

While low-carb diets can be beneficial for some, they are not a one-size-fits-all solution. For those experiencing unexplained fatigue, hair loss, or weight plateaus, thyroid dysfunction may be the hidden culprit. By understanding the delicate interplay between carbohydrates, stress hormones, and thyroid function, you can make informed dietary choices that support—rather than sabotage—your long-term metabolic health.

If in doubt, consult a functional medicine practitioner or endocrinologist to tailor your diet to your unique hormonal needs. Your thyroid works tirelessly to keep you energized and thriving—it’s worth nourishing it properly.