TSH: what is a good level?

Written by Steve Fayzulayev. Clinically reviewed by Naomi Fayzulayev, FNP-C, MSN.

Last reviewed 2026-11-09. See our editorial policy and methodology.

TSH reference intervals vary by laboratory, assay and population. Age, pregnancy, medications and health conditions also affect interpretation.

Longevity-optimised target 1–2.5 mIU/L

This is the range used by the hormonal dial in the Longevity Score calculator. It is deliberately tighter than the range printed on a standard lab report — the reasoning is below.

What your lab flags vs what the evidence supportsmIU/L
Standard
0.5–4.5
Optimised
1–2.5 mIU/L
02.55

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A result anywhere in the pale band is reported as normal and generates no comment. The brighter band is where risk is lowest. The distance between them is the part a standard report does not tell you.

What TSH actually measures

TSH is a pituitary hormone that stimulates the thyroid. In primary thyroid disease, TSH often changes in the opposite direction to thyroid hormone; pituitary disease and other circumstances can alter this relationship.

TSH is a sensitive initial test for many thyroid problems, but an individual result needs interpretation in context, sometimes with free T4.

Why it matters

Thyroid hormone sets the metabolic rate of essentially every tissue. Subclinical hypothyroidism — a raised TSH with normal free T4 — is associated with higher LDL, worse endothelial function and increased cardiovascular risk.

A suppressed TSH can occur with an overactive thyroid or excess thyroid medication and may be associated with atrial fibrillation and bone loss. Low TSH has other possible causes too.

Fatigue, cold intolerance, weight changes and poor concentration have many possible causes. A clinician can decide whether thyroid testing is appropriate.

Why it's on the dashboard

The hormonal dial includes TSH because both high and low results can be clinically relevant. Observational studies have associated some thyroid abnormalities with cardiovascular and fracture outcomes, but a calculator score does not diagnose thyroid disease or predict an individual outcome. See the Hormonal dial for the broader context.

Why this range is stricter than your lab's

The calculator's book-aligned 1.0–2.5 mIU/L scoring range is a model choice, not a diagnosis or treatment target. Clinical interpretation depends on laboratory ranges, age, pregnancy, medication and other circumstances.

A value outside the calculator range does not by itself establish thyroid disease or a need for treatment.

This distinction applies across the whole panel, not just this marker — see optimal versus standard lab ranges for the general case.

What moves TSH

Iodine sufficiency — in both directions
Deficiency and excess both cause thyroid dysfunction. High-dose iodine or kelp supplements are a common and avoidable cause of abnormal results.
Selenium
Selenium is involved in thyroid hormone metabolism. Supplements are not a substitute for thyroid care and excessive intake can be harmful.
Nutrition and illness
Severe illness or inadequate nutrition can affect thyroid tests; interpretation depends on the circumstances.
Ferritin and vitamin D
Both are commonly low alongside thyroid dysfunction and both affect symptoms independently.
Levothyroxine
The standard treatment where it is warranted. A clinical decision — and over-replacement carries its own bone and cardiac risk.

Educational information, not a treatment plan. Discuss changes to medication or supplementation with a clinician who knows your history.

How fast it responds

Six to eight weeks. TSH lags changes in thyroid hormone substantially, so retesting sooner measures the lag rather than the effect. This is the reason dose adjustments are conventionally spaced six weeks apart. See response times across all markers for how this compares.

What can distort the result

Score TSH against all 8 dials

Frequently asked questions

What is a good TSH level?

The longevity-optimised target used in this calculator is 1–2.5 mIU/L. TSH reference intervals vary by laboratory, assay and population. Age, pregnancy, medications and health conditions also affect interpretation.

Why is this target stricter than the range on my lab report?

The calculator's book-aligned 1.0–2.5 mIU/L scoring range is a model choice, not a diagnosis or treatment target. Clinical interpretation depends on laboratory ranges, age, pregnancy, medication and other circumstances.

How fast does TSH respond to change?

Six to eight weeks. TSH lags changes in thyroid hormone substantially, so retesting sooner measures the lag rather than the effect. This is the reason dose adjustments are conventionally spaced six weeks apart.

TSH feeds the hormonal dial, and read it alongside Testosterone · Ferritin · Morning Cortisol.

All biomarkers

Sources and further reading

These sources provide general context; calculator scoring ranges are not clinical treatment targets.

Not medical advice. These are longevity-optimised reference ranges, which are deliberately stricter than standard clinical "normal" values. A value outside them does not mean you have a disease. Speak to a qualified healthcare professional before changing medication, supplements or training. Full disclaimer.
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What is a good TSH level?

The longevity-optimised target used in this calculator is 1–2.5 mIU/L. TSH reference intervals vary by laboratory, assay and population. Age, pregnancy, medications and health conditions also affect interpretation.

Why is this target stricter than the range on my lab report?

The calculator's book-aligned 1.0–2.5 mIU/L scoring range is a model choice, not a diagnosis or treatment target. Clinical interpretation depends on laboratory ranges, age, pregnancy, medication and other circumstances.

How fast does TSH respond to change?

Six to eight weeks. TSH lags changes in thyroid hormone substantially, so retesting sooner measures the lag rather than the effect. This is the reason dose adjustments are conventionally spaced six weeks apart.

What is biological age?

Chronological age counts birthdays. Biological age estimates how old your body appears to be functioning, based on measurable biomarkers. Two people born the same year can differ by a decade or more in vascular stiffness, insulin signalling or cardiorespiratory fitness. Biological age makes that gap explicit — and unlike your birthday, it can move in either direction.

How is the Longevity Score calculated?

Each biomarker you enter is scored against longevity-optimised reference ranges and grouped into one of eight dials. Every dial produces its own biological age, and those are combined using fixed weights into a single composite, then expressed as a Longevity Score from 0 to 100. The weighting reflects how strongly each system predicts healthspan: structural and vascular carry the most, hormonal the least.

Do I need all 53 biomarkers?

No. The short form contains a subset of scoring inputs plus context-only fields, and you can leave any of them blank. It is not a recommendation to obtain tests. Missing values are excluded from the calculation — never treated as zero or as 'normal'. A dial with no scored data is dropped entirely and the remaining weights renormalise.

What can I measure without a blood test?

Some inputs can be measured without bloodwork. Blood pressure needs a home cuff and waist-to-height needs a tape measure. Wearable estimates vary; the floor-based sitting-rising test carries a fall risk and should only be attempted if safe.

Is this an anti-aging calculator? Is it a healthspan calculator?

Both terms describe what this tool does, and which one fits depends on what you are asking. "Anti-aging" is the term most people search, and loosely means anything aimed at slowing or reversing age-related decline — this tool fits that description if you score your dials and act on what you find. "Healthspan calculator" is the more precise term: healthspan is the number of years lived without serious chronic disease or disability, and that cannot be measured directly, so this tool scores eight systems that each function as an early-warning proxy for it. Either way, the method is the same: real biomarkers, published thresholds, no vanity-metric framing.

Why don't you have a quick quiz that skips biomarkers entirely?

Several popular tools ask only about habits — exercise days, sleep hours, smoking, diet quality — and return a single precise-looking age. We looked at the research behind that approach and chose not to build it. Published studies do show real, individually-measured links between specific habits and epigenetic age acceleration, but those effect sizes come from different study populations, different DNA-methylation clocks and different measurement units. Adding them together into one number is not something the underlying research actually supports — it produces a precise-looking result built on an imprecise method. Our dials use direct physiological measurements instead: a real blood pressure reading, a real VO₂ max, a real ApoB value. Several dials score from home-measurable data alone with no quiz-guessing involved — see what you can measure without a blood test, above.

Why are your 'optimal' ranges stricter than my lab's normal range?

Standard reference ranges describe what is common in a general population, which now includes a great deal of metabolic dysfunction. They are tuned to flag disease, not to identify what is optimal for a long healthspan. A fasting insulin of 20 µIU/mL sits inside most labs' normal range and is strongly associated with insulin resistance. This tool uses longevity-optimised thresholds instead, so a flag here means room for optimisation — not a diagnosis.

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