Thyroid monitoring
Levothyroxine blood tests: how often should you be checked?
Reviewed by a qualified clinician · analysed at UKAS-accredited UK labs (ISO 15189)
Last reviewed August 20266 min read
Every Helvy guide is written by our health editors, then checked by a qualified clinician before it goes live and re-checked as the science moves. We name clinical roles, not individuals, until each reviewer has agreed to be credited publicly. This is wellness guidance to help you understand your own data, not a diagnosis.
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NICE advises measuring TSH every 3 months after you start or change levothyroxine, until your level settles. Settled means two similar in-range results, 3 months apart. After that, once a year is the routine. If symptoms carry on, NICE says measure Free T4 as well as TSH.
You take the tablet first thing, every morning. Your last blood test came back normal. And you still feel flat, cold, and slower than you used to be.
So how often should your levels actually be checked? UK guidance gives a clear answer, and it is more specific than most people are told.
By the Helvy Medical Team · 6 min read
As of August 2026. Monitoring intervals reflect NICE guideline NG145, thyroid disease: assessment and management.
1. How often should you have a blood test on levothyroxine?
Levothyroxine is one of the most dispensed medicines in England. NHS Business Services Authority prescription data put it sixth, at 35 million items in 2025/26. Monitoring is still the part people are least clear about.
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
Two intervals sit inside that one sentence. Every 3 months while your dose is being settled. Once a year after that.
That applies to adults taking levothyroxine for primary hypothyroidism, the common form where the thyroid gland itself is underactive. NICE sets different intervals for children and young people.
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 does two jobs at once. 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 that climbs above the range suggests the current dose is no longer keeping up. One that drops below suggests the opposite. Neither reading 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 in a conversation with your GP. Dose decisions are theirs to make, not a test result’s.
3. Which markers should a thyroid check include?
Most NHS monitoring is a single number: TSH. That is the screening test, and for many people it is enough. Here is what each marker adds.
TSH
Thyroid stimulating hormone is the signal your pituitary gland sends to your thyroid. Read it as a thermostat. A high TSH suggests your body is asking for more thyroid hormone. A low one suggests it is getting more than it needs.
FREE T4
The main hormone your thyroid releases, in the portion free for your cells to use. NICE recommendation 1.4.4 says to consider measuring it as well as TSH if symptoms continue after starting levothyroxine. Our Thyroid & Vital Organs panel (£159) and General Energy & Wellness panel (£149) both measure Free T4.
FREE T3
The active hormone, converted from T4 inside your tissues. NHS labs often will not run it, because reporting rules stop at TSH when TSH looks normal. Our Thyroid & Vital Organs panel (£159) measures Free T3.
For all three in one sample, the Thyroid & Vital Organs panel (£159) measures TSH, Free T4 and Free T3, alongside kidney, liver and cholesterol markers.
Our TSH guide explains how to read the number itself, and normal thyroid levels sets out the UK ranges. The thyroid hub collects every guide in one place.
4. Why might you still feel tired when TSH is normal?
This is the most common frustration on levothyroxine. Your number is in range. You do not feel in range.
Sometimes the answer is the thyroid. Often it sits next to it. 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 (£159) measures iron, ferritin, transferrin, transferrin saturation, total iron binding capacity, B12, folate, vitamin D and magnesium.
Sleep, stress and low mood belong on the list too. None of them show up on a thyroid panel, and all of them wear people down.
Two more sit outside what we sell. A full blood count is a GP test, and is not part of any Helvy panel. Coeliac disease can cause 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 normal results and poor days are your pattern, our guide to normal results when you still feel ill goes further into why that happens.
5. Frequently asked questions
How often should TSH be tested on levothyroxine?
NICE advises every 3 months until your level has settled, then once a year. Settled means two similar results inside the reference range, taken 3 months apart.
How long should I wait after a dose change?
At least 6 weeks, per the NHS Specialist Pharmacy Service. Levothyroxine needs weeks to reach a steady level, and TSH responds more slowly still. An earlier test reads a number that has not settled.
Can I check my thyroid at home while on levothyroxine?
Yes, and many people do between NHS reviews. It sits alongside your GP monitoring rather than replacing it. Bring anything unexpected to your GP, and never adjust a dose yourself.
Should Free T3 be tested as well?
NHS labs rarely run it, because reporting rules stop at TSH when TSH looks normal. Some people want to see it anyway. Our Thyroid & Vital Organs panel (£159) measures TSH, Free T4 and Free T3 together.
Why do I still feel tired when my TSH is normal?
A normal TSH answers one question. Low iron stores, low B12, low folate and low vitamin D cause a very similar tiredness, and are all measurable. Our Vitamins & Minerals panel (£159) covers those four.
SEE THE FULL THYROID PICTURE
TSH, Free T4 and Free T3 in one sample
Our Thyroid & Vital Organs panel (£159) measures all three thyroid markers alongside your kidney, liver and cholesterol numbers. Home finger-prick kit, analysed at UKAS-accredited UK laboratories.
Medical disclaimer:This content is for informational purposes only and does not constitute medical advice, diagnosis, or treatment. Levothyroxine is a prescription medicine. Do not start, stop or change any medicine based on this article or on a home test result — consult your GP or a qualified healthcare professional. All Helvy blood tests are processed by UKAS-accredited UK laboratories to ISO 15189.
Last updated: August 2026 · By Helvy · Medically analysed at UKAS-accredited UK laboratories
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