Reading Thyroid Blood Tests Done in Peru: What Transfers and What Doesn't

A patient arrives with a printout from a laboratory in Miraflores. Her TSH is 4.8. In London, her last result was 4.1 and her GP told her it was normal. Now the Lima report has flagged it in bold. Has something changed, or has the standard?

Neither, as it happens. This is one of the most common sources of unnecessary alarm — and occasionally of unnecessary reassurance — among patients who arrive in Peru with a thyroid history. It is worth understanding properly.

The units transfer. The ranges don't always.

Start with the good news, because it removes most of the confusion.

TSH is reported in µIU/mL in Peru. That is numerically identical to mIU/L, the unit used across the UK, Europe and the US. A TSH of 2.1 in Lima is a TSH of 2.1 in Boston. There is no conversion to do.

Free T4 is usually reported in ng/dL in Peru, as it is in the US. If your previous results were in pmol/L — common in the UK and much of Europe — multiply the ng/dL figure by 12.87 to compare. A free T4 of 1.2 ng/dL is about 15.4 pmol/L.

Thyroid antibodies (anti-TPO, anti-thyroglobulin) are reported in IU/mL, but here the number genuinely does not transfer between laboratories. Different assays use different calibrations, and one lab's "under 35" is another lab's "under 60". What matters clinically is whether antibodies are present and clearly elevated, not the exact figure — and comparing an antibody titre from one lab to another is close to meaningless.

Where the reference ranges actually differ

The printed range at the right-hand edge of the report is not a universal constant. It is the range that particular laboratory derived from its own reference population using its own assay, and it varies — in Peru, in the US, and everywhere else.

For TSH, most Peruvian laboratories print something in the region of 0.4–4.5 µIU/mL. Some print 0.3–5.0. Some print 0.5–4.0. A result of 4.6 is therefore "high" at one lab and "normal" at another across the street, with your thyroid entirely uninvolved in the disagreement.

This is why the number alone does not decide anything. What decides is the number together with your free T4, your antibodies, your symptoms, your age, whether you are pregnant or trying to be, and what your previous results looked like. A TSH of 4.8 in a 60-year-old with no symptoms is a different clinical situation from a TSH of 4.8 in a 31-year-old planning a pregnancy, even though the printout looks identical.

If you want to see where your own result sits before your appointment, the TSH interpreter walks through the main scenarios — adult, pregnancy, and ongoing levothyroxine treatment, which each have different targets.

Three things that genuinely change your result

Before concluding that your thyroid has changed since you moved, rule these out.

When you took your levothyroxine. If you take it on the morning of the blood draw, your free T4 will be transiently higher than your true steady state. Take it after the blood draw, not before.

Biotin supplements. High-dose biotin — very common in hair and nail supplements — interferes with the immunoassays most laboratories use for thyroid testing. It can push TSH artificially down and free T4 artificially up, producing a picture that looks like hyperthyroidism in someone whose thyroid is fine. Stop biotin for at least 48 hours before testing, and tell whoever ordered the test that you were taking it.

A different levothyroxine brand. Formulations are not perfectly interchangeable, and switching brands — which frequently happens when you move countries — can shift your levels enough to matter. If your dose is unchanged but your TSH has moved, the brand is one of the first things I ask about.

What I actually want to see at the first appointment

If you are coming in with results from a Peruvian lab, bring:

• The full report, not just the TSH. Free T4 changes the interpretation entirely.
• Your previous results from abroad, even as photographs on your phone. One TSH tells me where you are. Three years of them tell me the direction of travel, and direction is usually more informative than position.
• The box of whatever you are taking, or a photo of it. Brand and dose.
• When you last took your dose relative to the blood draw.

The honest summary

Peruvian laboratories are perfectly good, the units are the ones you already know, and your thyroid did not change because your address did. What changes between countries is the reference range printed on the page and, sometimes, the brand in the box.

If a result here has been flagged and nobody has explained it to you, that is a reason to have it interpreted — not a reason to assume something has gone wrong. Most of the time the explanation is straightforward, and the visit ends with reassurance rather than a prescription.

If you are noticing symptoms alongside an abnormal result, this guide on when thyroid symptoms warrant a specialist covers what to look for.