Hormone Health Hub: Expert Insights on Testing, Balance & Better Living — reverse T3

Iodine is an essential trace element required for thyroid hormone production, yet many people are unknowingly deficient. While iodised salt and seafood are well-known sources, dairy products, eggs, and seaweed also contribute significantly to daily intake. Changing dietary habits — including reduced milk consumption and the replacement of iodine with bromine in bread production — are quietly lowering iodine intake across the population. This article explains where dietary iodine comes from, why the body needs it, how it is absorbed and excreted, and how at-home urine testing can confirm whether your intake is adequate.

Reverse T3 (rT3) is one of the most misunderstood markers in thyroid testing. Often called the “hibernation hormone,” it is frequently claimed — without scientific basis — to block thyroid receptors and slow metabolism. In Part 2 of this series, Margaret Groves of ZRT Laboratory examines what the peer-reviewed literature actually says about rT3, when elevated levels are clinically meaningful, and how to interpret the T3/rT3 ratio in the context of a comprehensive thyroid assessment.

Thyroid symptoms can persist even when thyroxine (T4) levels appear normal — because thyroid function is profoundly affected by anything that disrupts the conversion of T4 to the active hormone T3 at the cellular level. In Part 1 of this two-part series, Margaret Groves of ZRT Laboratory explains the three deiodinase enzymes (D1, D2 and D3) that act as gatekeepers to intracellular thyroid hormone bioavailability, what affects circulating levels of T3 and reverse T3, and why selenium, iodine and inflammatory conditions all play a critical role.

Normal thyroid blood tests don’t always explain why patients still suffer from fatigue, cold intolerance, brain fog and weight gain. In this in-depth clinical article, ZRT Laboratory explains the critical role of selenium in thyroid hormone synthesis and T4-to-T3 conversion, how heavy metals like mercury and arsenic sequester selenium and inactivate protective antioxidant enzymes, and why this can trigger Hashimoto’s thyroiditis — even when TSH, T4 and T3 appear normal.