Most patients are told their thyroid is “normal” simply because their TSH falls within range. Yet thousands continue to struggle with fatigue, weight gain, hair fall, constipation, low mood, infertility, or brain fog.
Why does this happen?
Because the thyroid system has two different levels of function:
- Central Thyroid Function – controlled by the brain
- Cellular Thyroid Function – how thyroid hormones actually work inside the cells
A person can have perfectly normal TSH (central function) but still have hypothyroidism at the cellular level, where metabolism actually happens.
This article breaks down the difference in a simple way, based on the thyroid physiology explained by Dr. Anish Musa during the FMCN Live Thyroid Masterclass.
1. The Thyroid System Starts in the Brain: Central Thyroid Function
Your thyroid gland responds to instructions from the brain through the HPT Axis (Hypothalamus–Pituitary–Thyroid Axis).
Step 1: Hypothalamus → TRH
The hypothalamus senses stress, inflammation, sleep, temperature, energy availability and releases TRH (Thyroid Releasing Hormone).
Step 2: Pituitary → TSH
TRH stimulates the pituitary gland to release TSH (Thyroid Stimulating Hormone).
TSH tells the thyroid gland how much hormone to produce.
This is why TSH represents central thyroid function — it measures how well the brain is controlling the thyroid, not how the thyroid hormones work inside the body.
2. The Thyroid Gland Produces T4, the Inactive Hormone
The thyroid manufactures mostly T4, which is simply tyrosine + four iodine atoms.
But T4 is not the active hormone. It is only a storage form.
3. The Real Work Happens Inside Cells: Cellular Thyroid Function
Once produced, T4 travels to tissues such as the liver, gut, muscles and brain. Here, it gets converted into T3, the active hormone.
T3 controls real metabolism
T3 enters your cells, attaches to nuclear receptors, activates DNA, and drives:
- energy production
- fat burning
- mood regulation
- digestion
- hair growth
- body temperature
All major thyroid functions occur because of T3, not T4.
4. When the Body Is Stressed → Reverse T3 Rises
Under stress, inflammation, infections, trauma, or severe calorie restriction, your body may convert T4 into Reverse T3 (rT3) instead of T3.
Reverse T3 acts like a metabolic brake. It blocks T3 from entering the cell.
This causes symptoms of hypothyroidism even when:
- TSH is normal
- T4 is normal
This scenario is called Cellular Hypothyroidism.
5. Central vs Cellular Hypothyroidism: The Key Difference
Central Evaluation (Conventional Approach)
- Only looks at TSH
- Sometimes T4
- If these look normal, the patient is told their thyroid is “fine”
But central thyroid function only reflects brain signaling.
Cellular Evaluation (Functional Medicine Approach)
Cellular thyroid function looks at:
- Free T3 – active hormone
- Reverse T3 – metabolic brake
- T3:T4 conversion ratio
This determines how much thyroid hormone is actually working inside the cell, where metabolism happens.
A person may have:
- Normal TSH
- Normal T4
- Low Free T3 or high rT3
→ This is cellular hypothyroidism.
This explains why many patients continue to suffer despite taking thyroid medication.
6. Real-World Example From the Lecture
Dr. Anish shared cases where patients taking thyroid medicine had:
- Normal or low TSH (central function normal)
- But low Free T3 (cellular function poor)
Even 200 mcg of T4 medication did not correct the cellular hypothyroidism.
This shows why focusing only on TSH does not solve the root problem.
7. Why Understanding Both Levels Matters
When practitioners evaluate both central and cellular thyroid health, they are better able to:
- Identify hidden hypothyroidism
- Connect symptoms with physiology
- Stop unnecessary dose escalations
- Personalize treatment based on how the body actually uses thyroid hormones
Functional medicine therefore shifts the focus from “Is TSH normal?” to “Is thyroid hormone working inside the cells?”
8. Summary: Central vs Cellular Hypothyroidism
| Function Type | What It Measures | Key Marker | What It Means |
|---|---|---|---|
| Central Thyroid Function | Brain regulation of thyroid | TSH | Normal TSH ≠ normal thyroid |
| Cellular Thyroid Function | Hormone action inside cells | Free T3, Reverse T3 | Determines real metabolism |
A patient can have:
- Normal central function
- Poor cellular function
→ This is cellular hypothyroidism, the root cause of unresolved thyroid symptoms.
Want to Learn How to Diagnose Thyroid Dysfunction Accurately?
The FMCN Certification Program teaches practitioners how to interpret TSH, Free T4, Free T3, Reverse T3, and thyroid antibodies with a functional medicine lens.
Inside the course you will learn:
- Why TSH alone is misleading
- How to identify low T3 states
- How to differentiate central vs cellular hypothyroidism
- How to address autoimmune thyroid conditions
- Real case studies with Indian patient data
👉 Explore the FMCN Course: https://vitaone.in/education
Empower yourself to treat thyroid dysfunction at its root—not just on paper, but at the cellular level where it truly matters.
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