Low T3 and Reverse T3 Patterns

Your TSH can be normal while your cells receive too little active thyroid hormone. We read free T3 and reverse T3 as a pattern and address conversion, stress, inflammation, fuel, and nutrients.

What you may be noticing

Fatigue, cold hands and feet, constipation, dry skin, low mood, slow thinking, and weight that will not move. Perhaps lightheadedness when you stand. Your TSH is normal. Your free T4 is normal. Yet you feel hypothyroid, perhaps even while taking thyroid medication.

Why it is rarely one thing

The thyroid makes mostly T4, a storage hormone. Enzymes called deiodinases convert T4 into active T3 in the liver, gut, muscle, and brain, or into reverse T3, an inactive form. Selenium sits at the active site of these enzymes, so mineral status shapes conversion directly. This conversion is the body deciding how much metabolic fire to allow.

I treat free T3 as the clearest window into thyroid hormone expression, because it is the form that enters cells and switches on genes. T3 supports mitochondrial energy production, heart rate and vascular tone, and autonomic balance. When free T3 runs low, many people notice fatigue, brain fog, and poor tolerance of standing, even when TSH looks reassuring.

During illness, calorie restriction, intense stress, and inflammation, the body shifts toward less T3 and more reverse T3. In acute illness this is considered adaptive. Not a broken thyroid, but a body choosing to conserve. The functional question is what keeps it in conservation mode. Chronic stress and elevated cortisol, ongoing inflammation, insulin resistance, under-eating, low selenium or iron, toxic burden, and certain medications all influence deiodinase activity. The interpretation of reverse T3 is debated, so it is read alongside the full picture rather than alone.

Patterns we look for

• Free T3, free T4, reverse T3, and their ratios.
• Selenium, zinc, and iron status.
• Cortisol rhythm and stress load.
• Inflammatory markers and chronic infections.
• Calorie and carbohydrate intake, including chronic dieting.
• Fasting insulin and blood sugar.
• Liver, bile flow, and gut health, where much conversion happens.
• Orthostatic tolerance and autonomic balance.
• Overtraining and inadequate recovery.

How I evaluate it

The comprehensive functional neurological exam assesses reflex timing, processing speed, balance, heart rate and blood pressure on standing, and autonomic indicators. These show how thyroid hormone is acting in your brain and body, not only in your bloodstream.

Functional labs may include TSH, free T4, free T3, reverse T3, thyroid antibodies, ferritin, selenium, zinc, a cortisol rhythm, fasting insulin, inflammatory markers, and a comprehensive metabolic panel. Labs drawn during acute illness, a crash diet, or intense training may show temporary changes, so results are read in context. Medication decisions are made with your prescribing physician.

How we address it

• Identify what keeps the body in conservation mode.
• Eat enough: adequate calories, protein, and carbohydrate for your activity.
• Replete selenium, zinc, and iron where low.
• Calm inflammation and address hidden infections and toxic burden.
• Open drainage and support liver and gut function, where conversion happens.
• Rebalance stress physiology with nervous system regulation and protected sleep.
• Coordinate with your physician about thyroid medication type where appropriate.

Your cells need the active signal, not only the stored one. When the body feels safe and supplied, it releases that signal more freely.

Book your Comprehensive Functional Medicine Eval/Neurological Exam today to discover what the contributing root causes are that are underlying your health concerns.