Your Thyroid Problem Is Actually A Conversion Problem

Your thyroid makes very little of the hormone your cells actually run on.

This is the part of thyroid physiology that most clinical practice skips over, and it is the reason so many people feel terrible while their labs come back normal.

Here is the full chain before we get into the details. Your hypothalamus signals your pituitary, your pituitary releases something called TSH, which is thyroid stimulating hormone, and TSH tells your thyroid how much hormone to produce. Your thyroid responds mostly by making T4, which is an inactive storage form of thyroid hormone. That T4 then travels through the bloodstream to your liver, your kidneys, and your muscle tissue, where enzymes called deiodinases convert it into T3, which is the active form that actually enters your cells and drives metabolism. Your cells use the T3, the drop in circulating hormone signals back to the pituitary, and the loop resets.

TSH measures one point in that chain. It tells you whether your pituitary thinks more thyroid stimulation is needed. It does not tell you how much T4 is being made, how well it is being converted, or how much active T3 is actually reaching your cells.

The American Thyroid Association guidelines currently recommend using TSH as the primary monitoring tool, without routine testing of free T3 or reverse T3. The reasoning is that TSH reflects the overall feedback loop. The problem is that when conversion is breaking down in the periphery, the pituitary may not see it clearly enough to drive TSH out of range.

A 2011 study published in PLoS ONE looked at patients who had their thyroid completely removed and were on standard levothyroxine replacement, which is synthetic T4. More than 20 percent of those patients had abnormal free T3 or free T4 levels even when their TSH tested normal. These are people with no thyroid at all, fully dependent on the conversion process, and one in five of them had a conversion problem that TSH alone would have missed entirely.

The conversion process itself depends on three types of deiodinase enzymes distributed across different tissues. Type 1 deiodinase is concentrated in the liver and kidney and handles a large share of the body's baseline conversion. Type 2 deiodinase is found in skeletal muscle and the brain, and it does something slightly different: it converts T4 locally, meaning the T3 it produces gets used right in the tissue where it was made. Between 70 and 90 percent of circulating T3 comes from this peripheral conversion process rather than direct thyroid secretion.

This is where skeletal muscle becomes metabolically important in a way most people do not consider. More muscle tissue means more sites running type 2 deiodinase, which means more local T3 production happening throughout your body. The relationship runs in the other direction too, because thyroid hormone status affects IGF-1 levels, which is one of the signals that drives muscle protein synthesis, so the two systems are in a feedback relationship with each other.

Several things disrupt the conversion process without necessarily moving TSH. Low dietary protein is one of them. Protein supplies the raw amino acids that support enzyme function across the liver and other tissues, and a 2025 case report in Cureus documented a patient with Hashimoto's thyroiditis who resolved her symptoms and saw meaningful improvement in thyroid antibody levels after moving to a high protein dietary protocol, without changing her medication. Selenium deficiency is another direct disruptor, because deiodinase enzymes are selenoproteins, meaning selenium is structurally built into the enzyme itself and the enzyme cannot function without it.

Cortisol is the one that cuts deepest, and it is also the most common. Chronic stress elevates cortisol, and chronically elevated cortisol suppresses deiodinase activity while simultaneously increasing the production of something called reverse T3, which is rT3. Reverse T3 is produced when T4 gets converted the wrong way, into a biologically inactive mirror image that cannot bind to thyroid receptors the way active T3 does. High rT3 effectively competes with active T3 for receptor space. The body is producing the hormone and converting it, but it is converting it into a form that blocks rather than activates the pathway. Standard labs do not include reverse T3. Many clinicians do not order it.

A 2025 cross-sectional study in Biomolecules looked at people with Hashimoto's thyroiditis who did recreational exercise compared to sedentary controls with the same diagnosis. The exercise group showed measurably lower levels of key inflammatory markers, including TNF-alpha and IL-6, which are the same inflammatory signals that suppress deiodinase function. Exercise was not fixing the thyroid directly. It was reducing the inflammatory environment that was disrupting conversion.

When lifestyle changes are not enough, the clinical evidence points toward direct T3 replacement rather than simply increasing T4 dose. A 2022 trial published in Frontiers in Endocrinology looked at female hypothyroid patients who still had significant symptoms despite normal TSH on levothyroxine. Adding liothyronine, which is synthetic T3, produced meaningful improvements in quality of life scores compared to continuing T4 alone. The mechanism makes sense: if conversion is impaired, giving the body more substrate to convert poorly does not solve the underlying bottleneck. Bypassing it does.

The TSH-plus-T4 framework was developed in the 1970s and it has largely held its shape since then. It works well for a straightforward thyroid problem where the gland is underperforming and conversion is intact. It does not map well onto a conversion problem, which is a different failure mode in the same system.

Most thyroid symptoms are not a production problem. They are a downstream problem, happening in the liver, the muscle, the stress response, and the diet, in places that a TSH number was never designed to see.

---

References

1. Gullo D, Latina A, Frasca F, Le Moli R, Pellegriti G, Vigneri R. (2011). Levothyroxine Monotherapy Cannot Guarantee Euthyroidism in All Athyreotic Patients. PLoS ONE 6(8):e22552. PMC3148220.
2. Cited for: the >20% of patients with abnormal FT3 or FT4 despite normal TSH on levothyroxine.
3. Limbaugh SL, Ennessy LA, Davis KS, Allen VT. (2025). Nutrition Based, High Protein Adjunct Therapy for Hashimoto's Thyroiditis: A Case Report. Cureus 17(9):e91597. PMC12495900.
4. Cited for: protein protocol resolving Hashimoto's symptoms.
5. Vuletić M, Žnidar V, Barić Žižić A, et al. (2025). Recreational Exercise and Inflammatory Patterns in Hashimoto's Thyroiditis: Observations from a Cross Sectional Study. Biomolecules 15(11):1510. PMC12650045.
6. Cited for: exercise reducing inflammatory markers in Hashimoto's patients.
7. Bjerkreim BA, Hammerstad SS, Gulseth HL, et al. (2022). Effect of Liothyronine Treatment on Quality of Life in Female Hypothyroid Patients With Residual Symptoms on Levothyroxine Therapy. Frontiers in Endocrinology 13:816566. PMC8902821.
8. Cited for: clinical evidence that adding T3 improves quality of life in patients with residual symptoms.
9. Lindner HH. (2025). Clinical thyroidology: beyond the 1970s' TSH T4 Paradigm. Frontiers in Endocrinology 16:1529791. PMC12234311.
10. Cited for: critique of the TSH only diagnostic model. Note: single author opinion piece.
11. European Thyroid Journal. Role of hepatic deiodinases in thyroid hormone homeostasis. PMC10160546.
12. Cited for: D1 distribution in liver and kidney, D2 distribution in skeletal muscle.
13. ScienceDirect Topics. Thyroxine Deiodinase.
14. Cited for: the 70-90 percent peripheral conversion figure.
15. Pituitary Foundation. Hormone Replacement Medication Interactions.
16. Cited for: thyroid hormone status affecting IGF-1 levels.
17. American Thyroid Association Guidelines on Treatment of Hypothyroidism.
18. Cited for: current guideline recommending TSH for monitoring without routine FT3 or rT3 testing.

---

Join the free community:
Men: Iron Forge Brotherhood
Women: Powerhouse Fitness