In 30 years of clinical practice, the single most common scenario I encounter is this: a patient comes to me with every classic symptom of hypothyroidism — fatigue so profound they can barely get through the day, weight gain that does not respond to diet and exercise, hair falling out in handfuls, brain fog so thick they cannot complete a sentence, depression, cold intolerance, constipation — and their doctor has told them their thyroid is "normal."
Their TSH is 3.2 mIU/L. By conventional standards, this is normal. The reference range says 0.5 to 4.5 mIU/L. They are well within range. Their doctor is not wrong, technically. But they are not right either — because "normal" and "optimal" are not the same thing, and in thyroid medicine, the gap between those two standards is the difference between a patient who is suffering and a patient who is thriving.
The Problem With TSH-Only Testing
TSH — thyroid stimulating hormone — is produced by the pituitary gland, not the thyroid. It is a signal from the brain to the thyroid, telling it to produce more thyroid hormone. When the thyroid is underperforming, the pituitary produces more TSH to compensate. When the thyroid is overperforming, TSH drops.
This makes TSH a useful screening tool. But it is only a screening tool. It tells you what the pituitary thinks about thyroid function. It does not tell you how much thyroid hormone is actually being produced, how much of that hormone is reaching the cells, or whether the hormone is being converted from its inactive storage form (T4) to its active form (T3) that the cells can actually use.
Testing TSH alone is like evaluating the performance of a car by asking the driver whether they are pressing the accelerator harder or softer. It gives you information about the demand signal, but nothing about what the engine is actually doing. To understand thyroid function completely, you need to measure the hormones themselves.
The Complete Thyroid Panel
A complete functional medicine thyroid evaluation includes six markers, each providing distinct and irreplaceable clinical information.
TSH (Thyroid Stimulating Hormone) is the pituitary signal. The conventional reference range is 0.5-4.5 mIU/L. My functional optimal range is 1.0-2.0 mIU/L. A TSH above 2.0 mIU/L, while technically "normal," indicates that the pituitary is working harder than optimal to maintain thyroid output — a sign of early thyroid insufficiency. A TSH above 3.0 mIU/L in a symptomatic patient is, in my clinical experience, almost always associated with significant hypothyroid symptoms.
Free T4 (Free Thyroxine) is the inactive storage form of thyroid hormone produced primarily by the thyroid gland. It must be converted to Free T3 to be biologically active. The conventional reference range is 0.8-1.8 ng/dL. My functional optimal target is the upper half of the reference range. Free T4 in the lower half of the reference range, combined with symptoms, indicates insufficient thyroid production.
Free T3 (Free Triiodothyronine) is the active form of thyroid hormone — the molecule that actually enters cells and drives metabolic function. This is the most clinically important thyroid marker, and it is the one most commonly omitted from conventional thyroid panels. The conventional reference range is 2.3-4.2 pg/mL. My functional optimal range is 3.2-4.0 pg/mL — the upper third of the reference range. A patient with Free T3 in the lower half of the reference range will have significant hypothyroid symptoms regardless of what their TSH shows.
Reverse T3 (RT3) is an inactive isomer of T3 — it occupies T3 receptors without activating them, effectively blocking the action of active T3. RT3 is produced when the body is under chronic stress, during caloric restriction, in the presence of inflammation, or when selenium is deficient. Elevated RT3 is one of the most common causes of hypothyroid symptoms in patients with "normal" TSH and T4. The conventional reference range is 9.2-24.1 ng/dL. My functional target is below 15 ng/dL. The Free T3 to Reverse T3 ratio (Free T3 in pg/mL divided by RT3 in ng/dL, multiplied by 100) should be above 20 for optimal thyroid function.
TPO Antibodies (Thyroid Peroxidase Antibodies) are the primary marker for Hashimoto's thyroiditis — the autoimmune condition in which the immune system attacks the thyroid gland. The conventional reference range is below 35 IU/mL. My functional target is below 10 IU/mL. Any detectable TPO antibodies indicate autoimmune activity against the thyroid, even if the level is technically "normal." Hashimoto's is present in approximately 90% of hypothyroid patients, yet it is routinely missed because TPO antibodies are not included in standard thyroid panels.
Thyroglobulin Antibodies (TgAb) are a secondary autoimmune marker. Some Hashimoto's patients have elevated TgAb but normal TPO antibodies — testing only TPO will miss these patients. My functional target is undetectable.
The Optimal vs. Normal Comparison
The table below illustrates the critical difference between conventional "normal" ranges and the functional optimal ranges I use in clinical practice. A patient whose values fall within the conventional normal range but outside the functional optimal range will typically have significant symptoms that their conventional physician cannot explain.
| Marker | Conventional Normal | Dr. Jay's Functional Optimal |
|---|---|---|
| TSH | 0.5–4.5 mIU/L | 1.0–2.0 mIU/L |
| Free T4 | 0.8–1.8 ng/dL | Upper half of range |
| Free T3 | 2.3–4.2 pg/mL | 3.2–4.0 pg/mL |
| Reverse T3 | 9.2–24.1 ng/dL | Below 15 ng/dL |
| TPO Antibodies | Below 35 IU/mL | Below 10 IU/mL |
| Thyroglobulin Ab | Below 1 IU/mL | Undetectable |
The Conversion Problem: When T4 Does Not Become T3
One of the most important and most overlooked aspects of thyroid function is the conversion of T4 to active T3. This conversion occurs primarily in the liver, gut, and peripheral tissues through enzymes called deiodinases. When this conversion is impaired, a patient can have adequate T4 but insufficient active T3 — and will have all the symptoms of hypothyroidism despite a "normal" TSH and T4.
The factors that impair T4-to-T3 conversion are among the most common conditions in modern life: chronic stress and elevated cortisol (the single most powerful suppressor of T3 conversion), insulin resistance, selenium and zinc deficiency (both essential cofactors for the deiodinase enzymes), gut dysbiosis and leaky gut (approximately 20% of T4 to T3 conversion occurs in the gut), caloric restriction and very low carbohydrate diets taken to extremes, chronic inflammation, and certain medications including beta-blockers, amiodarone, and lithium.
This is why a patient on levothyroxine (synthetic T4) may feel no better than before treatment — they are receiving the storage form of thyroid hormone but cannot convert it to the active form. In these patients, the addition of T3 (either as liothyronine or as desiccated thyroid extract, which contains both T4 and T3) is often transformative.
Hashimoto's: The Autoimmune Thyroid Epidemic
Hashimoto's thyroiditis affects an estimated 14 million Americans, the vast majority of whom are women. It is the most common autoimmune disease in the developed world and the most common cause of hypothyroidism. Yet it is routinely missed — not because it is difficult to diagnose, but because the diagnostic test (TPO antibodies) is not included in standard thyroid panels.
The clinical significance of identifying Hashimoto's — rather than simply treating hypothyroidism — is substantial. Hashimoto's is an autoimmune disease, which means the treatment approach must address the immune system, not just replace the thyroid hormone. The most important intervention for Hashimoto's is not thyroid medication — it is eliminating the triggers that are driving the autoimmune attack. The most consistently effective trigger to eliminate is gluten.
The connection between gluten and Hashimoto's is well-established in the functional medicine literature. Gliadin, the protein component of gluten, has a molecular structure that closely resembles thyroid tissue. In patients with intestinal permeability (leaky gut), gliadin enters the bloodstream and triggers an immune response. Due to molecular mimicry, this immune response cross-reacts with thyroid tissue, driving the autoimmune attack. Gluten elimination — strict, not casual — reduces TPO antibodies in the majority of Hashimoto's patients within 3-6 months.
What to Ask Your Doctor
If you suspect thyroid dysfunction and your doctor has only tested TSH, I encourage you to request a complete thyroid panel. The specific tests to ask for are: Free T4, Free T3, Reverse T3, TPO Antibodies, and Thyroglobulin Antibodies. If your doctor declines to order these tests, you have the option of ordering them directly through a direct-to-consumer lab service.
When you receive your results, do not simply accept "normal" as an answer. Ask where your values fall within the reference range. A TSH of 3.8 is "normal" — but it is in the upper quarter of the range, and in a symptomatic patient, it warrants further investigation. A Free T3 of 2.5 pg/mL is "normal" — but it is in the lower quarter of the range, and it is associated with significant hypothyroid symptoms.
The numbers matter. Where they fall within the range matters. And the clinical context — your symptoms, your history, your other hormonal markers — matters most of all.
— Dr. Jay Wrigley, NMD