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Dr. Cassie Smith Tells Dr. Axe Gut Health Drives Thyroid and Hormone Problems

On The Dr. Josh Axe Show, endocrinologist Dr. Cassie Smith tells Dr. Axe why gut health may drive thyroid and hormone problems.

By Dr. Josh Axe · November 6, 2026 · 2 sources · Understand

Photo: Pexels

On The Dr. Josh Axe Show, released November 5, 2026, dual board-certified endocrinologist Dr. Cassie Smith told Dr. Axe why thyroid problems often start in the gut.

Dr. Axe noted that around 80% of hypothyroidism is Hashimoto's thyroiditis, an autoimmune thyroid condition. Smith estimated that almost 50 million Americans may have untreated hypothyroidism, and said many are told they're fine because nobody checks free T3, reverse T3 or TPO antibodies beyond TSH.

Smith said birth control takes away natural hormones that certain gut bacteria depend on, letting harmful bacteria take over and release lipopolysaccharide, a compound that damages the gut lining and leads to leaky gut. She said new ferritin guidelines, released the Friday before the episode, recommend at least 50 for menstruating women, versus an old normal range of 16 to 150; she aims for 70 for thyroid health. She also said PCOS was reclassified in May 2026 as polyendocrine metabolic syndrome, with seven types.

Smith's central claim is that the enzyme deiodinase, which turns T4 into active T3, is downregulated by metformin, birth control, statins, PPIs and antibiotics, and by gut problems and high insulin. She said 93% of Americans have insulin resistance, so many people have normal T4 but not enough active T3. She said very small doses of GLP-1 drugs can help metabolically unhealthy people alongside lifestyle changes, but warned that higher doses risk muscle and bone loss.

These figures come from Smith's clinical experience and the guidelines she cited, not a study presented in the episode, so the prevalence numbers and the birth control link reflect her reported pattern rather than established population data.

Dr. Axe’s Take

Dr. Smith is right that chasing TSH alone misses most of what's going on. I'd add free T3, reverse T3, TPO antibodies and ferritin to any thyroid workup. For gut repair, I'd prioritize bone broth, fermented vegetables, pumpkin seeds and Brazil nuts. On GLP-1 drugs, I have no position beyond recommending the minimum dose possible, paired with a diet high in protein, fiber and nutrient-dense foods, quality vitamins and supplements, and ideally knowing your vitamin D, iron and B12 levels. Watch closely for muscle loss, the risk she raised.

— Dr. Axe

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Story summary

Who: Dr. Cassie Smith, dual board-certified endocrinologist, interviewed on The Dr. Josh Axe Show
Design: Podcast interview, reported clinical experience and cited guideline updates, not a clinical trial
Primary result: Smith estimates almost 50 million Americans have untreated hypothyroidism; Dr. Axe noted around 80% of hypothyroidism is Hashimoto's
Secondary: New ferritin guidance: at least 50 for menstruating women, versus an old range of 16-150; 93% insulin resistance cited; PCOS reclassified into 7 types as of May 2026
Date: Nov 5, 2026