How to track your TSH over time

TSH is the usual first test of your thyroid. It turns up on routine physicals, in workups for tiredness or weight change, and every few months for anyone taking thyroid medication. Most people end up with a long string of TSH results from different clinics and labs, each read on its own against the range printed next to it. That throws away most of what the test can tell you. TSH is a test where your own history matters more than usual, because your personal normal is much narrower than the lab’s.

What TSH measures

TSH stands for thyroid-stimulating hormone. It is not made by the thyroid. It comes from the pituitary gland, which uses it to tell the thyroid how much hormone to make. The thyroid’s main product is T4 (thyroxine), with a smaller amount of T3.

The two run in opposite directions. When thyroid hormone runs low, the pituitary sends out more TSH to push the thyroid harder. When there is too much, TSH falls. The NIDDK puts it simply: a high TSH most often means an underactive thyroid, and a low TSH usually means an overactive one.

Why TSH and free T4 are read together

Most T4 in the blood is attached to carrier proteins. The small unbound share, free T4, is the part your body can use, and it is the T4 value most often ordered next to TSH. Total T4 is also still reported by some labs. It shifts with the amount of carrier protein, which pregnancy and estrogen raise, so free T4 is usually the better one to track. Your cycle shows up in your labs covers how the pill affects total T4 while leaving TSH and free T4 mostly alone.

The relationship between the two is not a straight line. TSH responds to free T4 on a roughly logarithmic scale, so a small change in free T4 produces a large change in TSH. The American Thyroid Association’s hypothyroidism guideline calls this the source of the test’s sensitivity: TSH can be abnormal while T4 and T3 are still inside their ranges. That is why TSH is the main screening test, and why it swings more than free T4 does from one draw to the next.

The numbers on the report

TSH is reported in mIU/L or µIU/mL. These are the same number in different notation, so a TSH of 2.1 mIU/L is 2.1 µIU/mL. No conversion is needed. Some reports write “uIU/mL” in place of µIU/mL.

A typical adult reference range runs from about 0.4 to 4.0 or 4.5. The ATA guideline uses 0.4 to 4.0 mIU/L as an example, and many U.S. labs print something close to 0.45 to 4.5. The exact limits come from each lab’s own method and population, which is one reason a value near the edge can be flagged at one lab and not at another.

The terms you will see on a report or in a note:

Age and pregnancy

TSH tends to rise with age in people with no thyroid disease. In a large U.S. survey, Surks and Hollowell found the upper end of the TSH distribution (the 97.5th percentile) was 3.56 mIU/L for people in their twenties and 7.49 for people 80 and older. They concluded that without age-specific ranges, mild underactive thyroid is likely overdiagnosed in older adults. Most labs still print a single adult range.

Pregnancy lowers TSH, especially in the first trimester, when the pregnancy hormone hCG stimulates the thyroid directly. The 2026 ATA guideline on thyroid disease in pregnancy, which replaced the 2017 version this year, prefers lab- and trimester-specific ranges. When a lab does not have one, it suggests 0.1 to 4.0 mU/L for the first and second trimesters. If you have TSH results from a pregnancy, keep that context with them, or they will look like an odd dip in your history.

How often it is tested

How often TSH is checked depends on why:

• Screening or a normal result: on a schedule your clinician sets. It is often repeated only when symptoms or another condition call for it.
• A first abnormal result: usually repeated, with free T4, before anything is decided. Mild changes often do not hold up on a second test.
• After starting or changing levothyroxine: the ATA guideline recommends checking TSH 4 to 6 weeks after any dose change. That is how long levothyroxine takes to reach a steady level and TSH to settle.
• On a stable dose: less often, at intervals your clinician sets.
• Pregnancy on levothyroxine: much more often. The 2026 guideline describes testing about every 4 weeks until mid-pregnancy and at least once near 30 weeks.

The lag after a dose change is the part that matters most for reading a chart. A TSH drawn two weeks after a dose increase shows only part of the effect. If the dose was changed again before TSH had settled, the next result reflects a mix of both doses. Did the medication actually do anything? covers reading a before-and-after fairly.

What counts as a real change

TSH varies a lot within the same person, even when nothing has changed. The European Biological Variation Study, which sampled 91 healthy adults weekly, put the within-person variation of TSH at about 18 percent. Free T4 varied by about 5 percent. So free T4 is a steadier number from week to week, and TSH is the one more likely to move on its own.

At the same time, each person’s TSH sits in a narrow band of its own. In a year of monthly testing in healthy men, Andersen and colleagues found that each person’s range was about half as wide as the lab’s range. They also found that two TSH results had to differ by about 0.75 mU/L on average before the change was likely to be real, with the threshold anywhere from 0.2 to 1.6 depending on the person. Their conclusion was that a result inside the lab’s range is not necessarily normal for you.

Put together, that gives a few practical rules:

• A change of a few tenths is usually noise. A TSH of 1.8 followed by 2.2, with nothing else changed, is not news.
• A change bigger than your usual spread deserves a look. If your last five results ranged from 1.2 to 2.0, a 3.9 is a real move even though it is still “in range”.
• One high result followed by a return to normal is common. In the TRUST trial of older adults with mild subclinical hypothyroidism, average TSH fell from 6.4 to 5.5 over a year in the group taking a placebo. Several results moving the same way say more than one.
• Read free T4 alongside it. A TSH move with free T4 shifting the opposite way is more convincing than a TSH move alone.

Things that move TSH

Some TSH changes have nothing to do with your thyroid’s long-term state. Most are reasons to add a note, not to doubt the result.

• Time of day. TSH follows a daily rhythm, with a surge overnight. A study of nearly half a million TSH results found the reference interval varies with the hour of the draw. An early-morning draw and a late-afternoon draw are not quite comparable. If you can, have TSH drawn at a similar time of day each time.
• Biotin. Many immunoassays, including some TSH and free T4 tests, use a biotin-based chemistry, and biotin supplements, common in hair and nail products, can throw them off. In a small JAMA study, 10 mg a day for a week caused interference in some but not all such tests, with results falsely high or falsely low depending on the test design. A study in Thyroid at the same dose found that TSH and free T4 changed on one major lab platform but not another, with the largest effect a couple of hours after a dose. At high doses, the pattern can look like an overactive thyroid: low TSH and high free T4 that return to normal once biotin is held. Tell your clinician and the lab if you take biotin, and ask how long to stop before a draw.
• Illness and recovery. Serious illness can lower thyroid hormone levels without thyroid disease, and the NIDDK notes that steroids can too. As people recover, the ATA guideline notes, TSH can rebound above its usual level for a while. A TSH drawn in the hospital, or in the weeks after, may not reflect your baseline.
• How you take levothyroxine. Food, coffee, calcium, and iron all reduce how much levothyroxine you absorb. The ATA guideline suggests taking it consistently, either an hour before breakfast or at bedtime, and separating it from calcium and iron. A change in routine can move TSH as much as a dose change.
• Switching brands or generics. Different levothyroxine products are not always interchangeable. The ATA guideline suggests rechecking TSH after a switch.
• Weight change, aging, and pregnancy can all change how much levothyroxine you need.
• A change of lab. Labs use tests from different manufacturers and set their own ranges. The 2026 ATA pregnancy guideline notes large differences between free T4 methods in particular. A small step at the point you changed labs may be the method, not you.

Subclinical hypothyroidism

A TSH above the range with a normal free T4 is called subclinical hypothyroidism. It is common, especially with age, and it is where a history helps most. Whether to treat it is not settled. A 2019 BMJ guideline recommends against thyroid hormone for most adults with subclinical hypothyroidism, because trials found no meaningful benefit for symptoms or quality of life. It recommends monitoring instead. It does not apply to people trying to become pregnant or with a TSH above 20, and may not apply to young adults or people with severe symptoms. Monitoring means repeat TSH tests over time, which is what a chart is for.

An underactive thyroid also raises LDL cholesterol. If your LDL climbed without an obvious reason, look at your TSH from the same period. How to track your cholesterol over time has more on reading a lipid history.

What to record with each result

For each TSH, keep:

TSH goes by many names on reports: TSH, Thyroid Stimulating Hormone, Thyrotropin, TSH 3rd Generation, TSH High Sensitivity, and Sensitive TSH. Free T4 also appears as FT4, Free Thyroxine, and T4 Free. Pick one name per test and use it everywhere, or your history will split into short, separate lines.

Charting it

A few choices make a TSH chart honest:

• Draw your target, not just the lab range. If you are on levothyroxine, your clinician may aim for a narrower band than the lab’s range, or a different one in pregnancy or after thyroid cancer.
• Space points by date. A result 6 weeks after a dose change and one 2 years later should not look evenly spaced.
• Mark every dose change, and look at the first TSH 4 to 6 weeks after each one. Results drawn before that are partial.
• Chart free T4 alongside TSH, on its own axis. Expect them to move in opposite directions, with TSH moving much more.
• Consider a log scale if your TSH has ranged widely, for example from 0.1 to 10. On a plain scale, everything under 2 looks flat.
• Note anything unusual about a draw: illness, pregnancy, biotin, a new lab.

How to track your lab results over time has the general method for any test. If your older results are scattered across portals and clinics, How to get all of your medical records covers requesting them. A TSH from before you started treatment is often the most useful baseline you have.

Tracking TSH in HealthViewer

HealthViewer is a lab results tracker that runs on your own computer. It imports TSH results from lab report PDFs, MyChart downloads, and direct connections to your health system. It files TSH, Thyroid Stimulating Hormone, and TSH - Thyroid Stimulating Hormone under one test, including results coded as standard or high-sensitivity TSH from your health system. Results in mIU/L and µIU/mL share one chart. If one of your labs uses another name, such as Thyrotropin or TSH, 3rd generation, you can combine them with Merge in the Test Catalog.

In Trends, the Thyroid tag pulls TSH, free T4, T4, free T3, and T3 together, and the Thyroid Panel filter shows the tests from your thyroid panels. The overlay view puts TSH and free T4 on one chart. Medication markers show where levothyroxine started and stopped on the TSH chart. Enter each dose as its own period in Medications, and every dose change gets a marker, so you can see which results came 4 to 6 weeks after it. Set your target as a custom range in the Test Catalog, and the chart shades your target instead of the lab’s range.

The journal holds the context that is not in the lab data: a switch of levothyroxine brand, a stretch of biotin, an illness, a pregnancy, a change of lab. Open the Journal menu in Trends and choose Show journal to see those notes as markers on the chart.

Everything stays in an encrypted file on your computer, with no account and no cloud service. Getting Started walks through the first import.

This post is general information, not medical advice. Talk to your clinician about your TSH target and before changing any treatment.