Thyroid Truths: Beyond the Blood Test

Understanding T3, T4, and Why Low Thyroid Isn’t Always a Thyroid Problem

What Is the Thyroid and Why Does It Matter?

Your thyroid gland, a small butterfly-shaped organ at the base of your neck, plays a central role in your body's metabolism. It produces hormones that influence everything from your heart rate to how quickly you digest food. But the story of thyroid health is more complex than a simple “high” or “low” hormone reading on a blood test.

At the heart of this story are two hormones: T4 (thyroxine) and T3 (triiodothyronine).

Explore the real causes of low thyroid symptoms—T3, T4, conversion, bioavailability, and why your thyroid blood test might not tell the whole story.

T3 and T4: The Basics

• T4 is the main hormone produced by the thyroid—about 80–90% of what it makes.

• T3 is the active form of thyroid hormone. It binds more strongly to receptors and has a much more potent effect on metabolism.

But here's the catch: most T3 is not made directly in the thyroid. Instead, it's made elsewhere in the body, mostly by converting T4 into T3.

The Biochemistry: From T4 to T3

T4 is essentially a "prohormone"—a storage form with 4 iodine atoms. To become biologically active, one iodine atom must be removed. This happens via enzymes called deiodinases, primarily in the:

• Liver (major site)

• Kidneys

• Gut

• Muscle tissue

There are 3 types of deiodinase enzymes:

• D1 (Type 1) – major contributor to circulating T3 (liver, kidneys).

• D2 (Type 2) – ensures cells get T3 even when blood levels are low (brain, muscle, pituitary).

• D3 (Type 3) – deactivates T4 to reverse T3 (rT3), an inactive form often elevated under stress or illness.

Poor liver function, nutrient deficiency, or chronic stress can significantly impair this conversion—even when the thyroid is producing adequate hormone.

Bioavailability: It’s Not Just What You Take, But What You Use

T3 needs to be available to cells to be effective, which means:

1. Adequate levels in the bloodstream

2. Proper cellular uptake

3. Responsive receptor sensitivity

4. Resistance to being prematurely deactivated

This explains why someone can have “normal” blood T4 but still feel hypothyroid—because cellular T3 bioavailability is low.

Thyroid Medication: How Does It Work?

Most common thyroid meds (like levothyroxine, or synthetic T4) aim to replace low hormone levels. The body is expected to convert this T4 to T3 naturally.

However, if T4→T3 conversion is impaired, these medications may not resolve symptoms. In such cases, doctors may prescribe:

• Liothyronine (T3)

• Natural desiccated thyroid (NDT) – contains both T3 and T4

Taking T4 medication can suppress natural thyroid hormone production via negative feedback to the brain. This is expected and not harmful if conversion and cellular access are working well.

How to Support the Six Major Players in T4 to T3 Conversion

Understanding what influences your thyroid hormones is only half the story. Here's how to work with your body's natural mechanisms to improve thyroid conversion and function:

1. Liver Health: Enhance Detoxification Pathways

The liver is the main site of T4→T3 conversion.

Support tips:

• Drink plenty of water

• Eat bitter greens (rocket, artichoke, dandelion)

• Add sulfur-rich foods (garlic, onions, eggs)

• Include cruciferous veg (broccoli, kale)

• Minimise alcohol, processed foods and environmental toxins such as chemicals, plastics and air pollution.

Signs of trouble: brain fog, bloating after fatty meals, waking at 2–4 a.m., skin breakouts.

2. Stress & Cortisol: The Hormone Hijacker

Chronic stress redirects the body away from repair and conversion, increasing reverse T3.

Support tips:

• Practice breathwork or meditation

• Prioritise restorative sleep

• Move gently: walking, yoga, tai chi

• Spend time in nature daily

Signs of trouble: "wired but tired", 3 a.m. wake-ups, dependency on caffeine or sugar.

3. Nutrients: The Molecular Co-Factors

Enzymes that convert T4 to T3 need key nutrients:

• Selenium: Brazil nuts, sardines, eggs
  Deficiency signs: hair loss, low immunity, low T3

• Zinc: oysters, red meat, pumpkin seeds
  Deficiency signs: poor wound healing, white nail spots

• Iron: liver, red meat, molasses
  Deficiency signs: fatigue, breathlessness, pale skin

• Vitamin B12: liver, shellfish, eggs, nutritional yeast
  Deficiency signs: brain fog, pins & needles, low mood

Ask your GP for: full nutrient panel including ferritin, zinc, selenium, and B12—not just TSH and T4.

4. Gut Health: The Absorption Gateway

If your digestion is impaired, even the best diet or supplements won't help.

Support tips:

• Chew thoroughly

• Eat fermented foods (sauerkraut, kefir)

• Reduce processed foods and refined sugar

• Try bitters before meals

• Consider gut testing if symptoms persist

Signs of trouble: bloating, undigested food in stool, multiple nutrient deficiencies, low energy post-meals.

5. Medication Review: Unseen Interference

Some medications can inhibit T3 production or absorption of key nutrients:

• Beta blockers, steroids, SSRIs: block conversion

• Proton pump inhibitors (PPIs): reduce B12 absorption

• Oral contraceptives: increase thyroid-binding proteins

What to say to your doctor:

“Could we review my medications with thyroid metabolism in mind? I’ve read that some may affect hormone conversion or nutrient absorption.”

6. Inflammation & Chronic Illness: The Silent Blockers

Inflammation blunts the body's ability to convert and use thyroid hormone.

Support tips:

• Eat anti-inflammatory foods (omega-3s, berries, herbs)

• Stabilise blood sugar

• Screen for underlying infections if symptoms persist

Signs of trouble: chronic fatigue, joint pain, weight gain, allergies, mood shifts.

In Summary: It’s a System, Not a Switch

Thyroid health is not just about what your thyroid gland produces—it's about how your whole system processes, converts, and responds to those hormones. The answer isn’t always in a higher dose of meds. Sometimes it’s in improving the terrain—the metabolic and systemic environment that governs these pathways.

If you're on thyroid medication but still feel unwell—or if you're experiencing symptoms of low thyroid despite "normal" tests—it's time to look deeper. Osteopathy cannot claim to cure thyroid issues, but we can work with your body to support improved digestion, metabolism and hormone function. And we can work with you to better understand your condition and how your symptoms are being managed by your GP or other medical professionals. Book a consultation with us at The BodyWorx, and let's explore your whole system, not just one gland or one hormone.