Hormone Health Hub: Expert Insights on Testing, Balance & Better Living — deiodinase enzymes

The thyroid gland may be small, but it plays a monumental role in maintaining your body’s health. Located in the neck and shaped like a butterfly, this endocrine powerhouse regulates everything from metabolism to mood through the hormones it produces. Yet, thyroid hormone imbalances often go unnoticed—despite affecting millions. According to the American Thyroid Association, more than 12% of the U.S. population will develop a thyroid condition during their lifetime, and up to 60% of those affected may not even know it. These disorders are frequently misdiagnosed or overlooked, often dismissed as symptoms of stress, ageing, or other common conditions....

Many patients produce sufficient thyroid hormone yet still suffer from classic hypothyroid symptoms. Why? Because standard TSH testing misses the full picture. In this clinical guide, Jim Paoletti explains the functional approach to thyroid assessment — examining T4 production, TBG binding, T4-to-T3 conversion, cortisol interference, ferritin and vitamin D — and outlines exactly which tests are needed for a complete analysis.

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.