When "Normal Labs" Don't Mean Healthy
Maria, 42. Five years on levothyroxine. TSH at 2.1 mIU/L — textbook-perfect compensation. But Maria keeps gaining weight, can't focus, wakes up exhausted, her hair is falling out, and her skin is dry.
"Your labs are normal," says her endocrinologist. "Maybe you should see a therapist?"
This conversation repeats itself in clinics around the world. And the problem isn't Maria. The problem is how we treat hypothyroidism.
How the Thyroid Works
The thyroid gland produces two main hormones:
T4 (thyroxine) — the "storage" form, relatively inactive. Makes up ~80% of thyroid outputT3 (triiodothyronine) — the active form that actually works inside your cells. ~20% of outputYour cells cannot use T4 directly. It must be converted to T3 by specialized enzymes called deiodinases. This conversion happens in the liver, kidneys, muscles, and other tissues.
The Problem with Levothyroxine
Levothyroxine (Synthroid, Euthyrox) is synthetic T4. Only T4. The logic is simple: give the body T4 and it will convert it to T3 on its own.
For most patients, this works. But for 15-20%, it doesn't. Why?
Impaired T4-to-T3 conversion: selenium, zinc, or iron deficiency; chronic inflammation; stress; liver diseaseDIO2 gene polymorphism: approximately 16% of people carry the rs225014 variant, which reduces type 2 deiodinase activity. A study by Panicker et al. (2009, Journal of Clinical Endocrinology & Metabolism) showed these patients feel better on combination T4+T3 therapyIgnoring T3: standard practice monitors only TSH and sometimes free T4. Free T3 — the hormone that actually does the work — is often never checkedWhat Is NDT (Natural Desiccated Thyroid)
NDT (Natural Desiccated Thyroid) is made from desiccated porcine thyroid glands. It contains:
T4 and T3 in an approximately 4:1 ratioT2 and T1 — minor thyroid hormones whose roles are still being studiedCalcitonin — in trace amountsNDT has been in use since the 1890s — it is one of the oldest endocrine medications in medical history. Brand names include Armour Thyroid, NP Thyroid, Nature-Throid (US), and Thyroid-S (Thailand).
The Scientific Evidence
NDT is not alternative medicine. The evidence base includes:
Hoang et al. (2013, Journal of Clinical Endocrinology & Metabolism): a randomized, double-blind, crossover trial. 70 patients. NDT led to greater weight loss and was preferred by patients (49% vs 19% in favor of NDT vs levothyroxine)Panicker et al. (2009): patients with DIO2 polymorphism show better cognitive outcomes and wellbeing on combination T4+T3 therapyPeterson et al. (2018, Thyroid): systematic review — a subset of patients consistently prefer NDT or combination therapy with comparable safety profilesWiersinga et al. (2012, European Thyroid Journal): even with normal TSH, 10-15% of patients on T4 monotherapy have persistent complaintsWho May Benefit from NDT
NDT is not for everyone. It may be the optimal choice for:
Persistent symptoms on levothyroxine despite normal TSH (fatigue, weight gain, cognitive issues)Low free T3 with normal T4 — a sign of impaired conversionDIO2 polymorphism (genetic testing is available)Patients who "feel the difference" — subjective improvement on NDT is reproducible in studiesHow I Prescribe NDT in Practice
My approach includes a full thyroid panel — not just TSH:
TSH, free T4, free T3 — the essential triadAntibodies (anti-TPO, anti-TG) — to identify autoimmune thyroiditisReverse T3 (rT3) — a marker of impaired conversionSelenium, zinc, ferritin, vitamin D — cofactors for T4-to-T3 conversionThe transition to NDT is done gradually, with labs rechecked at 6 weeks. Dosing is individualized — the goal is symptom resolution with safe hormone levels, not a TSH number.
Important Caveats
NDT does not replace levothyroxine for everyone — most patients do fine on T4 aloneSelf-treatment is dangerous: excess thyroid hormones cause tachycardia, osteoporosis, and atrial fibrillationMonitoring is mandatory: regular TSH, fT3, fT4 checksNDT does not treat the root cause of autoimmune thyroiditis — that requires a comprehensive approach (selenium, vitamin D, anti-inflammatory nutrition)Your Next Step
If you've been on levothyroxine for years and symptoms persist, don't accept "your labs are normal, so everything is fine." Ask your doctor to check free T3, not just TSH. That single test may explain why you're not getting better.
In a consultation, we review your full thyroid panel, assess conversion efficiency, check cofactors, and determine whether a therapy change is warranted. For many patients, switching to NDT or combination therapy is the difference between "living in fog" and feeling normal again.
