When Your T3 Drops and Everyone Panics

Your thyroid panel comes back and your T3 is lower than it was before you started carnivore. Your doctor looks concerned. Someone in your carnivore Facebook group tells you to add carbs back immediately. Your functional medicine practitioner wants to prescribe thyroid medication.

I get it. Seeing that number drop is scary.

But here's what most people don't understand: lower T3 on carnivore isn't necessarily thyroid dysfunction. It's often metabolic adaptation. And there's a huge difference between the two.

What's Actually Happening to Your Thyroid

When you switch to carnivore, your body shifts from burning primarily glucose to burning primarily fat. This metabolic shift affects thyroid hormone conversion in predictable ways.

T3 (triiodothyronine) is your active thyroid hormone. Your body makes it by converting T4 (thyroxine) into T3. Insulin plays a role in this conversion process. When you're eating carbs and your insulin is higher, your body converts more T4 into T3.

On carnivore, your insulin drops significantly. This is good—it means your body is becoming more insulin sensitive. But it also means less insulin-driven T4-to-T3 conversion.

Research shows general patterns—your body may respond differently based on your unique health profile. In metabolic studies, participants on very low carbohydrate diets often show a 10-20% decrease in total T3 levels while maintaining normal thyroid function.

The key question isn't "did my T3 drop?" It's "am I experiencing symptoms of hypothyroidism?"

Thyroid Dysfunction vs. Metabolic Adaptation

Here's how to tell the difference.

Signs of actual thyroid dysfunction:

  • Persistent fatigue that gets worse over time
  • Cold intolerance (you're freezing when everyone else is comfortable)
  • Unexplained weight gain despite eating carnivore
  • Brain fog that doesn't improve after adaptation
  • Hair loss beyond normal shedding
  • Constipation that doesn't resolve
  • Depression or mood changes

Signs of healthy metabolic adaptation:

  • Energy levels improve or stay stable
  • Body temperature normal (around 98.6°F upon waking)
  • Body composition improving (losing fat, maintaining muscle)
  • Mental clarity sharp
  • Hair and skin quality good
  • Digestion normal
  • Mood stable or improved

If you have energy, normal body temperature, improving body composition, and no hypothyroid symptoms—your lower T3 is adaptation, not dysfunction.

The Reverse T3 Piece

This is where it gets interesting.

When your body is stressed (physically or psychologically), it produces more reverse T3 (rT3). Reverse T3 is basically inactive T3—it blocks thyroid receptors without activating them.

Standard thyroid panels usually don't measure reverse T3. So you might see low T3 on your panel, but if your rT3 is also low, that's actually fine. It means your body is efficiently using the T3 it makes.

High rT3 with low T3 is a different story. That suggests your body is converting T4 into inactive rT3 instead of active T3—usually because of chronic stress, inflammation, or metabolic dysfunction.

On carnivore, most people see rT3 normalize or decrease. The elimination of inflammatory foods reduces the stress signal that drives rT3 production.

What the Research Actually Shows

A 2007 study published in the Journal of Clinical Endocrinology and Metabolism looked at thyroid function on very low carbohydrate diets. Participants showed decreased T3 levels but maintained normal metabolic rate and thyroid-stimulating hormone (TSH).

The researchers concluded that lower T3 in the context of low insulin wasn't pathological—it was a normal adaptive response to reduced carbohydrate intake.

Another study from 2015 found that subjects on ketogenic diets had lower T3 but higher free T4 and normal TSH. Their bodies were simply converting less T4 into T3 because they didn't need as much active thyroid hormone when insulin was low.

Research shows general patterns—your body may respond differently based on your unique health profile. These studies suggest that thyroid adaptation on low-carb diets is normal, not problematic, as long as symptoms and other markers remain healthy.

When to Actually Worry

There are real thyroid issues that can develop on carnivore—though they're rare.

1. True hypothyroidism from nutrient deficiency

If you're only eating muscle meat and skipping organ meats, you might be missing selenium, zinc, and iodine—all critical for thyroid function. This is why I recommend eating liver once a week and occasionally including oysters or other seafood.

2. Exacerbation of pre-existing Hashimoto's

Some people with autoimmune thyroid disease see improvement on carnivore (elimination of gluten and other triggers). Others see worsening, especially in the first few months. If you have diagnosed Hashimoto's, work with your healthcare provider during the transition.

If you're taking medications or have been diagnosed with any medical condition, you need individualized medical oversight. Don't make changes without consulting your doctor.

3. Over-restriction leading to metabolic slowdown

If you're eating carnivore but severely restricting calories (eating less than 1200-1500 calories per day consistently), your body might genuinely slow down thyroid function as a protective response. Carnivore should be satiating, not restrictive.

What to Track Instead of Just T3

If you're concerned about thyroid function on carnivore, here's what actually matters:

1. Basal body temperature

Take your temperature immediately upon waking, before getting out of bed. Normal is around 97.8-98.6°F. Consistently below 97.5°F suggests potential thyroid issues.

2. Full thyroid panel (not just TSH)

Ask for: TSH, free T4, free T3, reverse T3, and thyroid antibodies (TPO and TG). The full picture matters more than any single number.

3. Symptoms, not just numbers

Energy, body temperature, digestion, mood, body composition, hair/skin quality. If all of these are good, your thyroid is probably fine regardless of what your T3 says.

4. Context matters

How long have you been carnivore? If you're less than 3 months in, you're still adapting. Give your body time to adjust before making changes based on one blood test.

When to Consider Adding Carbs Back

I'm not anti-carb. I'm pro-metabolic health. For most people, carnivore works brilliantly. But there are cases where adding small amounts of carbs might help thyroid function:

  • You're a woman with pre-existing thyroid issues who feels worse on strict carnivore after 6+ months
  • You're training intensely (high volume, high intensity) and experiencing persistent fatigue
  • You have documented low T3 with hypothyroid symptoms that don't improve after 3-6 months
  • You've been chronically under-eating and your metabolism has slowed

In these cases, adding honey, fruit, or white rice might help. But try other interventions first: eat more (especially protein and fat), prioritize sleep, reduce training volume, manage stress, add organ meats.

Medical conditions and medication interactions are complex. If you're under medical care, any dietary changes require your doctor's input.

The Bottom Line

Lower T3 on carnivore usually isn't a problem. It's your body adapting to a lower-insulin environment. As long as you feel good, have energy, maintain body temperature, and don't have hypothyroid symptoms, you're fine.

But if you're experiencing genuine hypothyroid symptoms—persistent fatigue, cold intolerance, brain fog, unexplained weight gain—get a full thyroid panel and work with someone who understands both thyroid function and metabolic adaptation.

Your thyroid is smarter than you think. Trust the symptoms more than the numbers.

I'm not a doctor. I've researched this deeply and worked with many people, but I'm not your doctor. If you have health conditions, take medications, or need specific guidance, talk to someone who knows your full medical picture. Everything I write is educational based on research and what I've seen work. Your situation might be different.