Introduction: thyroxine does not work in every third patient
According to cohort studies, 20–30% of patients receiving L-thyroxine replacement therapy at a biochemically adequate dose (TSH in the reference range) continue to have residual symptoms of hypothyroidism: fatigue, dry skin, weight gain, hair loss, brain fog, cold intolerance, tendency toward constipation, and reduced libido.
The standard cardiologist-endocrinologist response is: “TSH is normal; go home, the problem is not the thyroid.” But the problem is precisely in the thyroid axis: more specifically, thyroxine does not reach the cell in its active form.
T4 (thyroxine) is a prohormone. By itself, it has almost no activity at the thyroid hormone receptor. The active form is T3 (triiodothyronine), whose affinity for the nuclear TR receptor is 10–15 times higher than that of T4. T4 → T3 conversion occurs in the periphery, not in the thyroid.
Key idea of the md_pereligyn protocol: normal TSH + normal fT4 + symptoms of hypothyroidism + low fT3 = functional hypothyroidism. This is not rare and not “patient laziness.” It is impaired peripheral conversion, visible in laboratory testing but not accounted for by the standard algorithm.
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Physiology of T4 → T3 conversion
The conversion of T4 to T3 is catalyzed by three deiodinases, each with distinct biology:
▸D1 (type 1) — liver, kidneys. Produces most circulating T3; a selenium-dependent enzyme. ▸D2 (type 2) — brain, pituitary, muscle, brown adipose tissue. Local conversion for tissue T3; selenium-dependent. ▸D3 (type 3) — placenta, skin, fetal brain. Inactivates T4 into rT3 (reverse T3) and T3 into T2 — a “dead-end branch.”
All three deiodinases are selenoproteins. The enzyme active center contains selenocysteine. Without adequate selenium, the enzyme does not assemble or assembles incompletely.
Normal distribution: the thyroid secretes 80% T4 and 20% T3. Of this T4, peripheral conversion normally produces T3 — 30% (the physiological channel), and rT3 — 40% (physiological inactivation). The rest undergoes excretion and sulfation.
During stress, inflammation, or fasting, D2 switches toward D3 — conversion is directed into rT3. This is an evolutionary adaptation: during crisis, the body lowers metabolism. But with chronic stress, this switch can remain stuck in “winter” mode for years.
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Where conversion fails: four mechanisms
•Selenium deficiency — deiodinase is a selenoprotein. Serum selenium levels <90 mcg/L are associated with reduced D1 and D2 activity. In regions with selenium-poor soil (Northern Europe, Northern Russia, Ukraine, Poland), deficiency is a background state. Therapeutic target: 120–150 mcg/L. •Chronically elevated cortisol — glucocorticoids directly activate D3 and suppress D2 at the transcriptional level. Result: T4 is diverted into the rT3 dead end, tissue T3 falls, and TSH remains normal (because fT4 does not change). Triggers: chronic stress, sleep deprivation <6 h, irregular eating, overtraining. •Ferritin deficiency — TPO (thyroid peroxidase) and conversion itself depend on iron as a cofactor. With ferritin <70 ng/mL, conversion suffers even with normal hemoglobin. Target ferritin in women with hypothyroidism: 70–100 ng/mL, not the “lower limit of normal.” •Systemic inflammation — IL-6, TNF-α, and IL-1β induce D3 and suppress D2. Marker: hsCRP >3 mg/L. Sources: visceral obesity, IBS, leaky gut, chronic infection (EBV, H. pylori), periodontitis, undiagnosed autoimmune disease.
Often all four factors operate in parallel in the same patient. Treating with selenium alone while cortisol remains elevated and ferritin remains low gives only a partial effect — hence “I tried selenium, it did not help.”
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Markers: what to measure besides TSH
Standard screening (TSH + fT4) is blind to functional hypothyroidism. Expanded panel:
▸TSH (thyroid-stimulating hormone) — target 1.0–2.0 mIU/L on replacement therapy. The laboratory range 0.4–4.0 is too broad for symptomatic patients. ▸fT4 (free thyroxine) — target upper half of the reference interval (for example, 14–18 pmol/L with a reference range of 9–22). ▸fT3 (free triiodothyronine) — the main marker of tissue activity. Target range 4.0–7.0 pmol/L (or 3.5–6.5 with a reference range of 3.1–6.8). In functional hypothyroidism, fT3 is “pressed toward the lower limit.” ▸rT3 (reverse T3) — indicator of “dead-end” conversion. Target 9–24 ng/dL (or 0.14–0.54 nmol/L). >24 ng/dL suggests chronic stress or systemic inflammation. ▸fT3 / rT3 ratio — functional index. <0.2 (in nmol/nmol) indicates impaired conversion. >0.4 is normal. ▸Anti-TPO (thyroid peroxidase antibodies) — normal <35 IU/mL. Any elevation indicates a Hashimoto background, a common underlying contributor to symptoms. ▸Serum selenium — target 120–150 mcg/L. ▸Ferritin — target 70–100 ng/mL in women with hypothyroidism, 100–200 in men. ▸Zinc, serum iron, TIBC, transferrin saturation — expanded iron status. ▸Vitamin D 25(OH)D — target 60–80 ng/mL. ▸hsCRP — inflammation marker, target <1 mg/L. ▸4-point salivary cortisol (08:00, 12:00, 16:00, before sleep) — circadian rhythm; a single morning sample misses chronic stress.
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Holistic protocol for restoring conversion
Principle: repair the enzyme, do not increase the T4 dose. Simply escalating L-thyroxine while the conversion block persists raises rT3 even further — the paradox of “more T4 = less T3.”
### 1. Selenium — the foundation
▸Selenium (L-selenomethionine) 200 mcg/day in the morning on an empty stomach, for at least 8 weeks, then daily or 5 days per week. ▸Check serum level after 8–12 weeks, target 120–150 mcg/L. ▸Do not exceed 400 mcg/day — toxicity (hair loss, brittle nails, garlic odor). ▸In Northern Europe and regions with selenium-poor soil, lifelong nutraceutical supplementation is justified.
### 2. Iron to target ferritin
▸Iron bisglycinate 25 mg every other day with vitamin C 500 mg for absorption. ▸No dairy, tea, or coffee within ±1 hour of intake. ▸Target ferritin 70–100 ng/mL; monitor every 8 weeks. ▸If oral forms are not tolerated, intravenous iron in an inpatient setting.
### 3. Zinc, magnesium, vitamin D
▸Zinc (bisglycinate / picolinate) 15–25 mg in the evening — cofactor for T3 synthesis and D1 regulation. ▸Magnesium (glycinate / taurate) 300–400 mg in the evening — thyroid receptor cofactor. ▸Vitamin D3 4000–10000 IU to a level of 60–80 ng/mL, + K2 (MK-7) 100–200 mcg. ▸Iodine is a separate topic; high doses are contraindicated in an autoimmune thyroiditis background. See the article [Iodine and thyroid: 5 steps](/blog/iodine-thyroid-five-step-protocol).
### 4. Lowering cortisol
▸Sleep 7–9 hours, bedtime before 23:00. One hour of sleep loss shifts cortisol by 15–20%. ▸Morning light exposure 10 minutes during the first hour after waking — circadian rhythm reset. ▸Adaptogens (ashwagandha 600 mg) — cortisol normalization and rT3 reduction in small studies. ▸Breathing practices 10 minutes/day — parasympathetic activation. ▸Limit caffeine after 14:00.
### 5. Reducing systemic inflammation
▸Mediterranean / DASH dietary pattern — olive oil, fish, vegetables, legumes. ▸EPA + DHA 2 g/day — omega-3, IL-6 reduction. ▸Curcumin (liposomal) 500 mg 2×/day — anti-inflammatory. ▸Treatment of hidden infections when confirmed (H. pylori, EBV reactivation, dental caries, periodontitis). ▸Reduction of visceral fat — waist circumference <94 cm (men) / <80 cm (women).
### 6. When to add T3 directly
▸Liothyronine (L-T3) 5–10 mcg in the morning or split dosing (5 in the morning + 5 at lunch) — discussed with a physician when conversion deficiency is confirmed and symptoms persist for 3–6 months on the full protocol. ▸NDT (natural desiccated thyroid) — a T4 + T3 combination in a physiological 4:1 ratio. Details are in the article [Hypothyroidism and natural desiccated thyroid (NDT)](/blog/hypothyroidism-natural-desiccated-thyroid-ndt). ▸Check fT3, fT4, TSH after 6–8 weeks after starting T3-containing forms.
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What does NOT work (and why)
▸Blindly increasing the L-thyroxine dose when fT3 is low — more substrate for D3, more rT3, same symptoms. ▸Selenium monotherapy with ferritin 15 ng/mL — the enzyme will not assemble without iron; the effect is partial. ▸Iodine at >500 mcg/day with selenium deficiency — provokes autoimmune thyroiditis exacerbation through oxidative stress in the thyrocyte. ▸T3 without cortisol correction — risks worsening anxiety and provoking tachycardia. ▸Self-prescribing liothyronine without an expanded panel and monitoring — uncontrolled T3 increases the risk of arrhythmias and osteoporosis. ▸“Normal TSH = thyroid is fine” — in a symptomatic patient with low fT3, this is a working hypothesis of functional hypothyroidism, not a reason to close the case. ▸Vegan / low-protein diet without correction — zinc, iron, B12, and tyrosine deficiency (T4 substrate). Conversion suffers.
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When to seek care
▸TSH within reference range, fT4 within reference range, but fT3 <4.0 pmol/L or in the lower quartile of the reference range. ▸fT3 / rT3 ratio <0.2 (in nmol/nmol). ▸rT3 >24 ng/dL with normal TSH. ▸Residual hypothyroid symptoms on an adequate L-thyroxine dose for at least 6 months. ▸Suspected functional hypothyroidism without a formal diagnosis — symptomatic subclinical presentation. ▸Hashimoto background + progression of symptoms despite standard therapy. ▸Chronic stress, sleep disturbance, weight gain with a “normal” thyroid panel.
I run an expanded panel (TSH, fT4, fT3, rT3, anti-TPO, anti-Tg, selenium, ferritin, zinc, vitamin D, 4-point salivary cortisol, hsCRP) and create a personalized protocol for restoring conversion — without blindly increasing L-T4.
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Conclusion
T4 is a prohormone. The active hormone is T3. The conversion enzyme — deiodinase — is assembled from selenium, iron, and zinc, and works under conditions of normal cortisol and low inflammation.
When any of these four conditions is disrupted, the patient enters functional hypothyroidism: TSH is normal, fT4 is normal, fT3 is low, and symptoms persist. This is visible on the expanded panel and invisible on the standard panel.
Treatment is directed not at the L-T4 dose, but at restoring the conditions required for the enzyme to work: selenium, iron, zinc, vitamin D, cortisol reduction, inflammation reduction, and, when needed, adding T3 or NDT. One measured fT3 can spare the dose escalation.
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Sources
▸Bianco AC, et al. Paradigms of dynamic control of thyroid hormone signaling. *Endocr Rev* 2019;40:1000–1047. PMID 31033998 ▸Köhrle J. Selenium and thyroid. *Best Pract Res Clin Endocrinol Metab* 2009;23:815–827. PMID 19942156 ▸Wajner SM, Maia AL. New insights toward the acute non-thyroidal illness syndrome. *Front Endocrinol* 2012;3:8. PMID 22654852 ▸Salvatore D, et al. Thyroid hormones and skeletal muscle — new insights and potential implications. *Nat Rev Endocrinol* 2014;10:206–214. PMID 24322650 ▸Wiersinga WM. T4 + T3 combination therapy: an unsolved problem of increasing magnitude and complexity. *Endocrinol Metab* 2019;34:1–4. PMID 30912330 ▸Toulis KA, et al. Selenium supplementation in autoimmune thyroiditis. *Thyroid* 2010;20:1163–1173. PMID 20025778 ▸Mancini A, et al. Thyroid hormones, oxidative stress, and inflammation. *Mediators Inflamm* 2016;6757154. PMID 27051079
Related articles: [Iodine and thyroid: 5 steps](/blog/iodine-thyroid-five-step-protocol), [Hypothyroidism and natural desiccated thyroid (NDT)](/blog/hypothyroidism-natural-desiccated-thyroid-ndt).
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FAQ
Can thyroid function be assessed by TSH alone? No, not if symptoms are present. TSH is a marker of pituitary feedback, not tissue activity. Patients with residual symptoms on L-T4 need an expanded panel: fT4, fT3, rT3, anti-TPO, anti-Tg, selenium, ferritin, vitamin D, salivary cortisol.
What should be done if fT3 is low while TSH and fT4 are normal? This is a working hypothesis of functional hypothyroidism. Algorithm: check selenium, ferritin, zinc, vitamin D, cortisol, hsCRP. Correct identified deficiencies. After 8–12 weeks, repeat the expanded panel. If fT3 has not increased and symptoms remain, discuss T3 or NDT with a physician.
How quickly does selenium raise fT3? Clinically — 6–12 weeks at a dose of 200 mcg/day L-selenomethionine when baseline deficiency is present. Serum selenium rises within 8 weeks; fT3 rises more slowly because the enzyme must be synthesized, incorporated into the membrane, and build the T3 pool in tissues. Check at week 12.
Is rT3 dangerous by itself? rT3 is biologically inactive, but high rT3 is a marker of “winter” mode: chronic stress, inflammation, fasting, severe illness. rT3 falls in parallel with cortisol and hsCRP reduction — it is not treated in isolation.
When should T3 (liothyronine) or NDT be tried? After 3–6 months on the full protocol (selenium, iron, vitamin D, zinc, cortisol reduction) if low fT3 and symptoms persist. Starting low-dose T3 (5 mcg in the morning) or switching to NDT should be done only under physician supervision, with expanded panel monitoring after 6–8 weeks.
*This article is for informational purposes and does not replace medical consultation. Before starting any nutraceuticals, changing medication therapy, or undergoing diagnostic procedures, discuss the plan with your treating physician.*
