TSH 4.0 is “normal.” Why we want it under 2.
Half the patients we see have already been told their thyroid is “fine.” Most have only had a TSH, which the lab flags as normal anywhere from about 0.4 to 4.5 mIU/L. The math is wrong.
The “normal” range is a statistical band. The lab takes a sample of the general population (which includes a lot of undiagnosed thyroid dysfunction), draws a 95% confidence interval around the average, and calls that “normal.” It is a description of who walks into the lab, not a description of who feels well.
What optimal actually looks like
In functional medicine, the target TSH is roughly 0.5 to 2.0 mIU/L for most adults. A 2002 review in the Journal of Clinical Endocrinology and Metabolism found that 95% of healthy adults without thyroid disease have a TSH under 2.5. The American Association of Clinical Endocrinologists proposed lowering the upper end of the reference range to 3.0 years ago. Most commercial labs never updated.
The practical effect: a person with a TSH of 3.6 walks out with a clean bill of health, even though they feel cold, tired, are losing hair, and cannot drop the last 15 pounds. Their pituitary is shouting at the thyroid to produce more, and the thyroid is barely keeping up.
TSH is only the first marker, not the only one
A real thyroid workup runs six markers, not one. We always include:
- TSH – the pituitary signal
- Free T4 – the storage form of thyroid hormone
- Free T3 – the active hormone that actually drives metabolism
- Reverse T3 – the stress-shunt form; high when the body is downregulating
- TPO antibodies – the Hashimoto marker, often elevated years before TSH moves
- Thyroglobulin antibodies – the second autoimmune marker; catches what TPO misses
Roughly one in five “normal TSH but symptomatic” patients we see have elevated TPO antibodies. They have Hashimoto thyroiditis, an autoimmune attack on the thyroid that the standard workup completely missed. The thyroid is being slowly destroyed; the protocol changes everything.
What it costs to know
The full panel costs roughly $180 to $260 through Quest or LabCorp on cash pay; often partly covered by insurance through superbill. See the full panel breakdown for what else we run alongside it.
When TSH is the wrong question
If you have symptoms but your TSH is genuinely in the optimal range (under 2.0) and your Free T3 is decent (3.0 to 4.0 pg/mL), thyroid is probably not the story. We pivot to ferritin, cortisol rhythm, B12, and sex hormones. The point is to run the right test for the right question, not to assume the answer.
If your last lab said “TSH 3.8, normal” and you still feel off, the labs are not lying to you. They just answered the wrong question.