If you've had your thyroid checked and been told it's fine, I need you to read this. Because “fine” might mean your doctor tested one marker, it fell within a very wide reference range, and nobody looked any further.
That's what happened to me. For 15 years.
The standard thyroid test and what it misses
When your GP checks your thyroid, they almost always test TSH: thyroid stimulating hormone. TSH is produced by your pituitary gland. When your thyroid hormone levels are low, TSH rises to tell the thyroid to produce more. When levels are adequate, TSH drops.
It's a useful screening tool. But it has a significant limitation. TSH tells you what the pituitary is asking the thyroid to do. It doesn't tell you how much active thyroid hormone is actually available in your body.
Your thyroid primarily produces T4 (thyroxine), which is an inactive storage hormone. T4 needs to be converted into T3 (triiodothyronine) before your cells can use it. T3 is the active form. It's what drives your energy, your metabolism, your body temperature, your mood, your cognitive function, and your ability to maintain a healthy weight.
If your body isn't converting T4 into T3 efficiently, your TSH can look completely normal, your T4 can look completely normal, and you can still be functionally hypothyroid. Because the hormone your cells actually need isn't there in sufficient amounts.
“This is the test most doctors aren't running: Free T3.”
Why some people don't convert T4 to T3 well
The conversion of T4 to T3 happens throughout the body — primarily in the liver, gut, and muscles — via enzymes called deiodinases. Several things can impair this process.
Genetics
The DIO2 gene produces one of the key enzymes responsible for T4-to-T3 conversion. Certain variants of this gene reduce the efficiency of conversion. This is more common than most people realise. If you carry this variant, you can be on standard thyroid medication (levothyroxine, which is synthetic T4) for years and still feel terrible — because your body can't adequately convert it into the form it needs.
Nutrient deficiencies
The conversion process requires selenium, zinc, iron, and iodine. Deficiencies in any of these can slow it down. These are common, particularly in women, and rarely checked alongside thyroid panels.
Chronic stress
Under stress, the body preferentially converts T4 into reverse T3 (rT3), an inactive form, rather than active T3. This is a protective mechanism designed for short-term survival — but when stress is chronic, it means less active thyroid hormone available to your cells.
Inflammation
Chronic low-grade inflammation interferes with deiodinase activity and can reduce T3 production.
Liver and gut health
Since a significant portion of T4-to-T3 conversion happens in the liver and gut, impaired function in either organ can directly affect thyroid hormone availability.
My story: 15 years with the wrong test
I was diagnosed with an underactive thyroid at 27. Put on levothyroxine immediately. Nobody asked about my lifestyle, my diet, my stress levels, or anything else. Just pills and a follow-up appointment.
For the next 15 years, my doctors checked my TSH regularly. It was always in range. The medication was “working.” Case closed.
But I was exhausted. Gaining weight. My liver enzymes were elevated for over a decade. I couldn't sleep. My mood was flat. I kept going back, kept being told I was fine, and kept feeling worse.
At 42, I started doing my own research. I discovered that some people don't convert T4 to T3 efficiently and that genetic variants can cause this. I got my Free T3 tested privately. It was low. Genetic testing confirmed I carry a variant that impairs T4-to-T3 conversion.
I found a private doctor who prescribed T3 directly (liothyronine) alongside my existing T4 medication. Within weeks, my liver enzymes started normalising for the first time in a decade. My sleep improved. My energy came back.
One test. One additional medication. 15 years of “normal” results explained.
The cascade effect of low T3
What made my situation particularly frustrating is that low T3 doesn't just cause fatigue. It creates a cascade of problems that can look like completely separate conditions.
01
Fatty liver disease
Low T3 slows metabolism, which can contribute to fatty liver disease. My elevated liver enzymes weren't caused by alcohol or diet. They were caused by my thyroid.
02
Insomnia
Low T3 impairs sleep quality. I spent years struggling with insomnia and was offered sleeping pills rather than a thyroid investigation.
03
Mood & cognitive function
Low T3 affects mood, cognitive function, and body weight. I was told I was stressed, ageing, or not trying hard enough. None of that was the problem.
04
Weight that won't shift
When T3 is low, your metabolic rate slows. No amount of calorie restriction or exercise will fully compensate for a thyroid that isn't delivering the hormone your cells need.
Every doctor looked at each symptom in isolation. Nobody connected them. And nobody tested the one marker that would have tied it all together.
What to ask for
If you have symptoms of low thyroid function — fatigue, weight gain, brain fog, low mood, hair loss, sensitivity to cold, dry skin, constipation — but your TSH has come back normal, consider requesting:
Free T3
This is the active hormone. If it's low or low-normal while your TSH is fine, that's a significant finding.
Free T4
This shows how much inactive hormone is available for conversion. A normal T4 with a low T3 suggests a conversion problem.
Thyroid antibodies (TPO and TG)
These can reveal autoimmune thyroid disease (Hashimoto's), which is the most common cause of hypothyroidism and can cause fluctuating symptoms even when TSH looks normal.
Reverse T3 (rT3)
If you want the full picture, this shows whether your body is converting T4 into the inactive form rather than the active one. This is often elevated in chronic stress and illness.
If your GP won't order these tests, private thyroid panels are accessible and affordable. A full panel including Free T3, Free T4, TSH, thyroid antibodies, and sometimes reverse T3 can be done for well under £100.
A note on medication
I want to be clear: I'm not anti-levothyroxine. It's an essential medication for millions of people and it works well for most. But “most” isn't everyone. For those who don't convert T4 to T3 efficiently, T4-only medication leaves a gap that no amount of dose adjustment will close.
If you've been on levothyroxine for years and still don't feel well, a conversation with your doctor about adding T3 (liothyronine) or switching to a combination approach may be worth having. This is a medical decision that should be made with a prescribing doctor, not a health coach. But knowing the right questions to ask is half the battle.
I'm not a doctor. I don't prescribe or diagnose. But I spent 15 years not asking the right questions because I didn't know they existed. That's the gap I try to help people close — starting with a 90-minute Initial Health Assessment that reviews your full history, existing results, and the patterns a standard GP appointment doesn't have time to explore. If you're wondering what that involves or what it costs, the services page has the detail.
You deserve more than a single number and a “you're fine.” The full picture exists. You just have to know to ask for it.
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