Reading thyroid labs over time: TSH, free T4, free T3 and antibodies

Thyroid results are easier to read as a series than one panel at a time. A single TSH moves with the time of day and the lab that measured it, so the direction across several draws, read with free T4, tells you more than any one number. Antibodies are different: they mark thyroid autoimmunity, guidelines don't ask for them to be repeated, and if you do follow them, only results from the same assay can be compared.

This one is personal. Sera started with my own Hashimoto's tests, scattered across years without the big picture. Each result made some sense on its own page. Together, from different labs in different units, they didn't add up to anything I could read. I build Sera, a lab-review workspace for practitioners, so keep that in mind for the last section.

What each marker adds

Thyroglobulin antibodies sometimes appear on the same panel. Their cut-off differs between platforms too, as the Tg antibody range page explains.

Why one TSH result says little on its own

The limits depend on the lab. The American Thyroid Association describes the traditional TSH reference range as 0.5 to 4.5 mIU/L. Laboratories print slightly different limits for their own assay, from about 0.3 to 0.45 at the bottom and 4.2 to 5.3 at the top. A result of 4.8 can be flagged high on one report and printed as normal on another.

TSH has a daily rhythm. It runs highest overnight and in the early morning. An 8am draw and a 3pm draw from the same person can differ for that reason alone.

One result isn't enough for a diagnosis. NICE's thyroid guideline, NG145, only considers treating subclinical hypothyroidism when TSH is above the reference range on two separate occasions three months apart. That is a guideline for doctors, but the principle holds for anyone reading the results: a single high TSH is a reason to look again.

Narrower targets are targets. Some practitioners aim for TSH below 2. That is a functional target with no guideline behind it. In pooled cohort data on 134,000 adults, the lowest cardiovascular and mortality risk sat at a TSH of roughly 1.9 to 2.9, not below 2. All of these figures, with their sources, are on the TSH range page.

Free T4 and free T3: check the unit and the lab

Free T4 and free T3 are assay-dependent. Intervals differ between testing platforms, and between countries the unit changes as well:

• Free T4: US laboratory intervals for adults are about 0.9 to 1.7 ng/dL (Mayo) and 0.82 to 1.77 (Labcorp). UK laboratories on Beckman assays report 8.0 to 18.0 pmol/L.
• Free T3: Labcorp gives 2.0 to 4.4 pg/mL. UK laboratories report about 3.7 or 3.8 to 6.0 or 6.6 pmol/L.

So a free T4 of 14 and a free T4 of 1.1 can both be ordinary results, in different units. Before you put two results on the same line, check they're in the same unit, and read each one against the interval its own lab printed. For free T4, pooled cohort data found the lowest cardiovascular and mortality risk in the low-normal part of the range, around 13.5 to 14.8 pmol/L, rather than the upper half. Optimal bands for both markers are functional targets with no published threshold behind them. Details are on the free T4 and free T3 pages.

How antibodies are usually followed

A positive TPO antibody result marks thyroid autoimmunity and is the usual antibody finding in Hashimoto's thyroiditis. Two things make the series hard to read.

The cut-off depends on the manufacturer. Laboratories on Beckman Coulter assays report positive from 9 IU/mL. Labcorp's adult range runs to 34 IU/mL. UK laboratories on the newer Roche kit use 20. A result of 25 can be positive on one assay and normal on another, so read each result against the cut-off on its own report.

The number isn't comparable between laboratories. A fall from 300 at one lab to 150 at another may not be a fall at all.

Guidelines and practice differ on whether to repeat the test. NICE NG145 says to consider measuring TPO antibodies when TSH is above the reference range, and not to repeat the test. Many practitioners in functional and integrative practice still track them, often once a year in Hashimoto's. If you follow them, keep to the same lab and the same assay where you can, and treat a change of lab as the start of a new series. The cut-offs and their sources are on the TPO antibody page.

What to record next to each draw

Most of the noise in a thyroid series comes from things nobody wrote down. Next to each result, record:

• Date and time of the draw. Because of TSH's daily rhythm, comparable draws are taken at a similar time of day.
• Lab and unit, exactly as printed. For antibodies, also the positivity cut-off printed on the report.
• Thyroid medication: the name, the dose, and whether that morning's dose was taken before or after the blood test. A dose just before the draw raises free T4.
• Biotin. NICE notes that a high intake of biotin from supplements can falsely raise or lower thyroid results, so ask about supplements by name.
• Changes since the last draw: diet, supplements, medication, and anything else the client thinks is relevant, with the date each one started.

With those notes, a jump in a chart can be read for what it is. Without them, a lab change or an afternoon appointment looks just like a real change.

Reading the series

Put the results in date order, one row per result, with the notes above. Then:

1. Group results by lab and unit. Compare like with like first.
2. Read TSH with free T4 from the same draw, not on its own.
3. Look at the direction over three or more draws before reading much into one.
4. Mark when anything changed, so you can see what came before and after.
5. Take a result outside the lab's own range, or a client on thyroid medication whose results have moved, to the clinician who manages their care.

Where Sera fits

Sera shows results over years and across labs against optimal and sufficient bands, with up to three markers on one chart, so TSH, free T4 and free T3 can sit together, with antibodies on a chart of their own. It keeps every result in the unit the lab reported and flags a marker that arrives in more than one unit. Every band has its sources and evidence grade published. Diet, supplement and medication changes are marked on the trend charts, Sera points out recognized patterns across markers such as a Hashimoto's antibody pattern, and Sera suggests a retest date for any result out of range and a yearly one for TSH, free T4, free T3 and TPO antibodies. It doesn't diagnose or recommend treatment.

It's free for three clients with no card. There's a 3-minute tour if you'd rather watch first.

Alina Radu is the founder of Sera and a software engineer. The idea started with her own Hashimoto's tests, scattered across years without the big picture.