Here’s a question almost no one gets answered at the pharmacy counter: if the medication is working, why don’t I feel like it’s working?
The answer comes down to two letters and one step between them. Your medication is T4. Your body runs on T3. And the step that turns one into the other is where a lot of people quietly get stuck.
T4 is the storage form. T3 is the active form.
- T4 (thyroxine) — the storage hormone. Stable, abundant, and mostly inactive. This is what levothyroxine gives you.
- T3 (triiodothyronine) — the active hormone. This is what actually tells your cells to make energy, keep you warm, think clearly, and burn fuel.
T4 is essentially a prohormone — a reserve that has to be activated before your body can use it (Bianco et al., 2019). When you take your pill, you’re topping up the storage tank. Whether that storage actually becomes usable energy depends entirely on the next step.
The conversion step happens mostly outside your thyroid
The majority of your active T3 is converted from T4 out in your body’s tissues — your liver and other organs do much of this work (Peeters & Visser, Endotext). Roughly 40% of the T4 you produce each day gets converted to T3 through this peripheral process (Bianco et al., 2019).
The conversion is done by enzymes called deiodinases. Here is the single most important fact in this article:
Deiodinases are selenoproteins. They are selenium-dependent enzymes. Without enough selenium, the conversion of T4 into active T3 cannot run at full capacity (Larsen & Zavacki, 2013).
When conversion stalls, your body makes the wrong hormone
There’s a third enzyme — D3 — that turns T4 into reverse T3 (rT3) — a biologically inactive mirror-image molecule (Bianco, 2018). Under stress, illness, or nutrient depletion, your body can shift toward making more reverse T3.
This is how you end up with a “normal” T4, a “normal” TSH, and a body that feels like it’s running on fumes — because the activation step is where it broke down.
Why this is the “missing half” your prescription doesn’t cover
Levothyroxine fills the storage tank. But replacing storage hormone does nothing to guarantee that you have the selenium your conversion enzymes need, or that your body isn’t shunting T4 toward inactive reverse T3.
This is why Thyrolume is built around a clinically meaningful dose of selenium (L-selenomethionine) to support the body’s own conversion machinery. It’s the difference between more fuel and a working engine.
👉 Next: Selenium & Your Thyroid: What 200mcg Does That 10mcg Can’t
👉 Or zoom out: Why You’re Still Exhausted Even When Your Labs Are “Normal”
Frequently asked questions
What’s the difference between T4 and T3? T4 is the inactive, storage form of thyroid hormone (and the form in levothyroxine). T3 is the active form your cells actually use. Your body must convert T4 into T3 to feel the benefit (Bianco et al., 2019).
Why isn’t my T4 converting to T3? Conversion is run by selenium-dependent enzymes (deiodinases) located mostly outside the thyroid. Selenium depletion, inflammation, stress, and illness can all slow it down and push your body toward making inactive reverse T3 instead (Larsen & Zavacki, 2013).
What is reverse T3? Reverse T3 (rT3) is a biologically inactive mirror-image of T3 that your body can make from T4 under stress or illness. High rT3 can mean you have hormone present but little of it in usable, active form (Bianco, 2018).
Does selenium help T4-to-T3 conversion? The enzymes that perform the conversion are selenoproteins, so adequate selenium is required for them to function. That’s the biochemical basis for selenium’s role in thyroid health — separate from its evidence for lowering thyroid antibodies.
