Levothyroxine blood tests: how often should you be checked?

1. How often should you have a blood test on levothyroxine?

Levothyroxine is among the most dispensed medicines in England. NHS Business Services Authority prescription data put it sixth, at 35 million items in 2025/26. Monitoring is the part people are least clear about.

Where your free T4 should sit, and whether age changes that, is covered in free T4 levels by age.

Most of those prescriptions start with a raised TSH. Our guide to high TSH levels covers the thresholds behind that decision.

NICE sets the interval out plainly.

“For adults who are taking levothyroxine for primary hypothyroidism, consider measuring TSH every 3 months until the level has stabilised (2 similar measurements within the reference range 3 months apart), and then once a year.”

— NICE guideline NG145, recommendation 1.4.3

That is for adults on levothyroxine for primary hypothyroidism, the common form where the gland itself is underactive. Children have different intervals.

Timing matters after a dose change. The NHS Specialist Pharmacy Service advises testing from 6 weeks, then every 3 months. Levothyroxine takes weeks to reach a steady level, and TSH lags behind it. Test too early and you read a number that is still moving.

2. What does a settled thyroid level mean?

NICE defines it inside the recommendation itself: two similar results within the reference range, 3 months apart. One good result is not enough. Two that agree are.

After that, an annual TSH is the routine. It checks whether the dose still fits you, and whether the tablets are going down as prescribed.

Why keep checking once you feel fine? Because dose needs drift. A TSH above the range suggests the dose is no longer keeping up, and one below suggests the opposite. Neither is a diagnosis. Each is a prompt for a GP conversation.

Some situations call for a check sooner than a year. A dose change. Pregnancy, or planning one. A large change in weight. Or starting something that gets in the way of absorption.

That last one catches people out. The NHS advises taking levothyroxine 30 to 60 minutes before your first meal, any drink with caffeine, or any other medicine. Calcium and iron supplements and indigestion remedies can all interfere.

Any of those belongs with your GP. Dose decisions are theirs, not a test result’s.

3. Should you take your tablet before the blood test?

Take it as normal, and leave a gap if you can. Never skip a dose to get a tidier result.

The reason is free T4, the thyroid hormone your body has available to use. The tablet lifts it for a few hours. Blood taken in that window shows a peak rather than your usual level. Our guide to normal free T4 levels sets out where the ranges sit.

“Ideally thyroid function blood tests should be done at least four hours after the morning dose of levothyroxine i.e. in the afternoon.”

— Leeds Teaching Hospitals NHS Trust

Leeds wrote that for people treated for thyroid cancer. The reason it gives is not specific to them. A high free T4 can show up when blood is taken soon after the tablet.

So, in practice:

• Leave four hours if free T4 is being measured. An afternoon appointment does this without any effort.
• Say when you last took it. Whoever reads the result can then read it properly.
• Do not skip the dose. If you are worried about timing, ask your GP or pharmacist rather than missing one.

TSH is the steadier of the two, which is why NHS monitoring leans on it. If your check is TSH only, the hour matters far less. Our TSH blood test guide covers what that one number can and cannot settle, and free T4 covers the other half.

4. Which markers should a thyroid check include?

Most NHS monitoring is one number: TSH. For many people that is enough. Here is what each marker adds.

For all three in one sample, that is the Thyroid & Vital Organs panel. The thyroid hub collects every guide in one place.

5. Why might you still feel tired when TSH is normal?

Your number is in range. You do not feel in range. It is the most common frustration on levothyroxine.

Thyroid symptoms overlap heavily with other measurable things, so a normal TSH settles one question, not all of them.

The usual suspects are nutritional. Low iron stores, low B12, low folate and low vitamin D all produce a tiredness that feels much the same. Our Vitamins & Minerals panel measures all four, plus the rest of the iron studies.

Sleep, stress and low mood belong on the list too, and none show up on a thyroid panel.

Two more sit outside what we sell. A full blood count is a GP test. Coeliac disease causes the same flat, drained feeling, and our coeliac testing guide covers how it is checked.

One boundary matters. A blood test measures your markers. It does not diagnose a condition, and it does not replace NHS monitoring. Take an unexpected result to your GP, who can repeat it and act on it. Never change a dose on the strength of a home test.

If that is your pattern, our guide to normal results when you still feel ill goes further.

6. Frequently asked questions