- How the thyroid system actually works
- What TSH does and does not tell you
- The free T3 and reverse T3 problem
- What "normal range" actually means
- The symptom picture of suboptimal thyroid function
- What a complete thyroid evaluation looks like
- Frequently asked questions
"Your TSH is normal" is the most common phrase in thyroid medicine, and one of the most clinically limiting. It is accurate as far as it goes: TSH (thyroid-stimulating hormone) is an effective screening marker for overt thyroid pathology — unambiguous hypothyroidism or hyperthyroidism at the population level. It misses a clinically significant subset of patients whose thyroid function is impaired enough to produce symptoms without crossing the diagnostic threshold.
Understanding why requires looking at how the thyroid system actually works — because TSH measures only one node in a multi-step pathway, and it is not the node where the most clinically relevant information lives.
How the thyroid system actually works
The thyroid gland produces two primary hormones: thyroxine (T4) and triiodothyronine (T3). T4 is the storage form — produced in large quantities by the thyroid and circulated to peripheral tissues. T3 is the active form — the hormone that actually binds thyroid receptors and drives metabolic effects in cells. T4 is converted to T3 primarily in the liver, kidneys, and other peripheral tissues by deiodinase enzymes.
TSH is produced by the pituitary gland. It stimulates the thyroid to produce T4. When T4 levels are adequate, TSH falls; when T4 is low, TSH rises. This negative feedback loop is what TSH screening measures — how well the pituitary perceives thyroid output.
The problem is that TSH reflects the pituitary's perception of circulating T4. It does not directly measure:
Whether T4 is being adequately converted to T3 in peripheral tissues.
Whether the T3 being produced is the active form (T3) or the inactive form (reverse T3, or rT3).
Whether cellular thyroid receptor sensitivity is normal.
What TSH does not tell you
A patient can have normal TSH, normal T4, and still have functional hypothyroidism at the tissue level through two primary mechanisms:
Impaired T4-to-T3 conversion. The deiodinase enzymes that convert T4 to active T3 are inhibited by several conditions: chronic stress and elevated cortisol, selenium deficiency, iron deficiency, chronic illness, certain medications, and — importantly — insulin resistance. A patient with multiple of these conditions may produce and circulate adequate T4 while converting it to active T3 inefficiently. TSH and T4 are normal; free T3 is low-normal or low.
Elevated reverse T3. Under conditions of physiological stress, the body converts T4 preferentially to reverse T3 (rT3) rather than active T3. Reverse T3 binds thyroid receptors without activating them — it is a competitive inhibitor of active T3. Elevated rT3 produces functional hypothyroidism at the tissue level while TSH and T4 remain normal. This is the pattern commonly described in chronic illness, HPA axis dysregulation, and severe caloric restriction.
The free T3 and reverse T3 problem
Free T3 — the unbound, biologically active fraction of T3 — is the most direct marker of thyroid hormone available to tissues. It is also the least commonly ordered in standard thyroid screening.
The significance of measuring free T3 alongside TSH is illustrated by two scenarios:
Patient A: TSH 2.1 (normal), free T4 1.1 (normal), free T3 2.1 pg/mL (low-normal, lower quartile of reference range). This patient is "normal" on standard screening but has low-normal active thyroid hormone. If symptomatic, this deserves clinical attention.
Patient B: TSH 2.1, free T4 1.1, free T3 2.1, reverse T3 28 (elevated). The rT3:T3 ratio indicates that available thyroid hormone is being competed out by reverse T3 at the receptor level. Tissue hypothyroidism is present despite normal TSH and T4.
A normal TSH is not the same as optimal thyroid function.
The comprehensive hormonal evaluation at Revitalize includes free T3, reverse T3, and the full thyroid panel — not just TSH.
Check Your Hormone SymptomsNeither of these patients would be identified by TSH screening alone.
What "normal range" actually means
Thyroid reference ranges — like all laboratory reference ranges — are derived from the population distribution of healthy adults. The standard TSH reference range (0.45 to 4.5 mIU/L in most labs) captures approximately the 95th percentile range of a reference population.
Several considerations limit the clinical usefulness of this range for individual optimization:
The reference population includes individuals who are subclinically hypothyroid — the range is wide enough that a proportion of "normal" individuals are symptomatic at their TSH value.
The optimal TSH for most adults — associated with best symptom control — is generally considered to be between 1.0 and 2.0 mIU/L. A TSH of 4.2 is technically normal but is associated with symptoms in a meaningful subset of patients, particularly women.
The same TSH value means different things in a 35-year-old woman and a 65-year-old woman — aging changes the relationship between TSH and thyroid output.
The symptom picture of suboptimal thyroid function
Thyroid hormone drives the metabolic rate of every cell in the body. Suboptimal thyroid function produces a predictable, consistent symptom cluster:
Persistent fatigue independent of sleep quality. Mental slowing — described as cognitive dulling, slower processing, reduced verbal fluency. Weight gain or difficulty losing weight despite appropriate caloric intake and exercise. Cold intolerance — particularly cold extremities. Hair thinning. Constipation. Dry skin. Low mood and reduced motivation. Bradycardia in more significant cases.
This symptom cluster overlaps significantly with testosterone deficiency, cortisol dysregulation, iron deficiency anemia, and depression. A complete evaluation distinguishes the contributors.
What a complete thyroid evaluation looks like
A clinically complete thyroid evaluation at Revitalize includes:
TSH — for context and to identify significant pathology.
Free T4 — the storage form; indicates thyroid gland output.
Free T3 — the active form; the most direct indicator of available thyroid hormone at the tissue level.
Reverse T3 — to identify competitive inhibition of active T3 by the inactive form.
TPO antibodies (anti-thyroid peroxidase) — to identify autoimmune thyroid disease (Hashimoto's), which is the most common cause of hypothyroidism and produces a distinct clinical picture.
These five markers, interpreted together and in the context of the patient's symptoms, provide a complete thyroid picture that TSH alone cannot offer.
Medical disclaimer: This article is for educational purposes only. Thyroid evaluation and treatment is a clinical decision. Consult a qualified healthcare provider before making treatment decisions.
Travis spent 17+ years in high-acuity clinical medicine — emergency, cardiac ICU, and cath lab — before founding Revitalize. He is a Certified Platinum Biote hormone therapy provider, the published author of You're Not Broken — You're Unbalanced, and the founder of the Rebuild Metabolic Health Institute. His clinical writing reflects the same precision he brought to critical care: specific, honest, and built around what actually works.
