- 01On levothyroxine, a TSH is read against its draw date
TSH answers for the dose you took about 6 weeks ago, not the one you take now, because levothyroxine has a 7-day half-life. A TSH drawn 2 weeks after a change mostly reflects the old dose, so the same 5.8 mIU/L means "still settling" at week 2 and "this is where the new dose landed" at week 8.
If you take levothyroxine (synthetic T4, the main hormone your thyroid makes), your TSH result tells your prescriber whether the dose fits, but only once the body has had about 6 weeks to adjust to that dose. TSH (thyroid-stimulating hormone) comes from the pituitary gland. It rises when the pituitary senses too little thyroid hormone and falls when it senses too much. Levothyroxine has a half-life of about 7 days (the time it takes for half of a dose to clear from the blood) 1, so thyroid guidelines advise rechecking TSH about 4 to 8 weeks after starting the drug or changing the dose 7. A TSH drawn 2 weeks after a change mostly reflects the old dose.
Levothyroxine builds up slowly. With a half-life of about 7 days, each daily tablet tops up a large pool of hormone in the blood, and that pool takes roughly five half-lives, about 5 to 6 weeks, to reach a new steady level after a dose change 1. The pituitary then adjusts TSH to match that new level. TSH therefore changes over weeks, and a draw taken too early lands partway between the old dose and the new one.
This is why the useful read on levothyroxine is the TSH trend across properly timed draws. A TSH of 5.8 mIU/L two weeks after a dose increase and a TSH of 5.8 mIU/L eight weeks after it mean different things: the first is still moving, and the second is where the new dose has landed. The general case for reading results as a series is in why trends beat single readings; the levothyroxine-specific part is the 6-week settling lag.
How often TSH is checked on levothyroxine
| Situation | When TSH is usually rechecked | Source |
|---|---|---|
| Starting levothyroxine | About 4 to 8 weeks after the first dose | AACE/ATA 2012 7; ATA 2014 1 |
| Any dose change, brand or manufacturer switch | About 4 to 8 weeks after the change | AACE/ATA 2012 7; ATA 2014 1 |
| Starting or stopping a drug that affects absorption or binding (for example oral estrogen, a proton pump inhibitor, iron or calcium) | Often 4 to 8 weeks after, at the prescriber's discretion | ATA 2014 1 |
| Stable dose, TSH in target | About 6 months, then about once a year | ATA 2014 1 |
| Pregnancy confirmed | Promptly, then about every 4 weeks through mid-pregnancy | ATA 2017 pregnancy guideline 3 |
TSH is a routine blood test. It does not require fasting, and it is often ordered together with free T4. What each line on the report measures is covered in how to read your thyroid panel; this page covers how those lines behave once you take levothyroxine for hypothyroidism. New to lab reports? Start with how to read any lab result.
TSH, free T4 and free T3 on levothyroxine
Free T4 and free T3 measure the share of each hormone that is not bound to carrier proteins in the blood, the part the body can actually use.
| TSH | Free T4 | Free T3 | |
|---|---|---|---|
| Role on levothyroxine | Main dose guide in primary hypothyroidism (the thyroid itself is the problem) 1 | Supporting check; becomes the main guide when the pituitary is the problem (central hypothyroidism), because TSH is then unreliable 1 | Rarely used for dose decisions; levothyroxine is T4 and the body converts it to T3 1 |
| Speed of response to a dose change | Slow, about 6 weeks to settle | Faster, days to weeks | Variable |
| Shifted by taking the pill shortly before the draw | Barely | Yes, can read higher for several hours 1 | Little |
| Shifted by high-dose biotin | Can read falsely low on some lab test methods 2 | Can read falsely high on some lab test methods 2 | Can read falsely high on some lab test methods 2 |
Four things can shift a thyroid result on levothyroxine while the underlying condition stays the same: when you took the pill relative to the draw, how well the tablet was absorbed, other drugs and life changes (such as oral estrogen, a brand switch or pregnancy), and biotin in the test tube.
1. Pill timing before the draw
Free T4 rises for several hours after a levothyroxine dose, while TSH barely moves over the same hours 1. Someone who swallows the tablet at 7 am and has blood drawn at 9 am can see a higher free T4 than the same person drawn before the dose. Neither result is wrong, but two results taken under different conditions are hard to compare. Ask your prescriber which way they want it done, and keep it the same for every draw.
2. Absorption: food, coffee, calcium, iron and acid-lowering drugs
Levothyroxine is absorbed in the stomach and small intestine, and several common things reduce how much gets in. The ATA guideline describes taking it consistently on an empty stomach, for example about 60 minutes before breakfast or at bedtime several hours after the last meal, and separating it from calcium and iron supplements by about 4 hours 1. In a randomized crossover trial, taking levothyroxine 60 minutes before breakfast gave lower and steadier TSH values than taking it with breakfast 6. Coffee, soy and high-fiber meals taken close to the tablet also reduce absorption 1. Spacing advice for iron supplements has its own detail on the iron page.
How levothyroxine is usually taken, per the ATA guideline
Stomach acid helps dissolve the tablet. In a study of 248 people with goiter (an enlarged thyroid) on levothyroxine, those with reduced stomach acid needed higher daily doses to reach the same TSH 4. The causes of low acid in that study were Helicobacter pylori (a common stomach bacterium), atrophic gastritis (long-term thinning and inflammation of the stomach lining), and the acid-lowering drug omeprazole. Proton pump inhibitors (PPIs, acid-lowering drugs such as omeprazole and pantoprazole) are widely used, so mention starting or stopping one to the prescriber who manages your thyroid dose.
3. Other drugs and life changes
Oral estrogen (in some birth control pills and menopausal hormone therapy) raises the protein that carries thyroid hormone in the blood, which can push TSH up and increase the dose a person needs 1. Switching between levothyroxine brands or generic manufacturers, a large change in body weight, and missed doses can also move TSH 1. Pregnancy is the biggest of these. Levothyroxine needs rise in early pregnancy, and the ATA 2017 guideline advises that the dose be raised by about 20 to 30% once pregnancy is confirmed, for example as two extra doses a week, with thyroid labs checked promptly 3.
4. Biotin in the test tube
Many thyroid tests (lab test methods, also called assays) use a biotin-based chemistry. Large amounts of biotin, at the levels found in many hair, skin and nail supplements, can interfere with those tests 2. On a thyroid panel the typical pattern is a falsely low TSH and a falsely high free T4, which looks like overtreatment when nothing about the thyroid has changed 2.
Biotin in blood or other samples taken from patients who are ingesting high levels of biotin in dietary supplements can cause clinically significant incorrect lab test results.US Food and Drug Administration, Safety Communication on biotin interference, 2017 (updated 2019) 2
Lab reports flag TSH against a reference range of roughly 0.4 to 4.0 mIU/L (milli-international units per liter, the standard TSH unit; see lab units explained). Each lab sets its own range, and some run to 4.5. For most adults on levothyroxine, the ATA's target is a TSH inside that range 1. The tiers below show what each band usually means once someone is on treatment, and the evidence behind the risk at the low end. How reference ranges are built, and why they vary, is covered in reference ranges explained.
| TSH (mIU/L) or situation | What the report usually says | What it can mean on treatment | Evidence |
|---|---|---|---|
| 0.03 or below | Suppressed | More hormone than the body needs, unless TSH is being suppressed on purpose (some thyroid cancer care) | Linked to more heart rhythm problems, heart disease and fractures in long-term users 5 |
| About 0.04 to 0.4 | Low, below range | Mildly more than needed, or a timing or biotin artifact; context decides | Not linked to excess heart or fracture risk in the same study 5 |
| About 0.4 to 4.0 | In range | The usual target for most adults | ATA 2014 1 |
| Upper part of range, or a little above, in people over about 70 | May be flagged high | Prescribers often accept a higher TSH in older adults to avoid overtreatment | ATA 2014 1 |
| About 4 to 10 | High | Dose may be short of need, absorption may have changed, or the draw came too soon after an increase | ATA 2014 1 |
| Above 10 | High | Usually undertreatment or poor absorption; worth raising with the prescriber promptly | ATA 2014 1 |
| During pregnancy | Trimester-specific ranges | ATA aims for the lower part of the trimester range, or below 2.5 if the lab has no pregnancy range | ATA 2017 3 |
Example: a TSH of 0.3 mIU/L on levothyroxinemIU/L
Why a low TSH on levothyroxine matters
A low TSH on treatment means the pituitary senses more thyroid hormone than it wants. Over years, too much thyroid hormone speeds the heart and increases bone turnover. The ATA guideline links overtreatment to atrial fibrillation (an irregular, often fast heart rhythm) and to bone loss, with the highest risk in older adults and postmenopausal women 1. In a Scottish cohort of 17,684 people on long-term levothyroxine, a TSH of 0.03 mIU/L or below was linked to more heart disease, heart rhythm problems and fractures, while a TSH between 0.04 and 0.4 was not 5. That gap is why a slightly low value and a fully suppressed value are read differently. Bone effects connect to osteoporosis and the wider bone topic.
A high TSH on treatment usually means not enough hormone is reaching the body. Before reading it as a dose problem, check the clock and the routine. Was the draw at least 4 weeks after the last change? Has anything changed in the routine, such as when the tablet is taken, a new calcium or iron supplement, a new acid-lowering drug, or a new brand? Those answers help your prescriber decide between a dose change and a routine fix. Symptoms of too much thyroid hormone overlap with hyperthyroidism, and symptoms of too little with hypothyroidism.
If your TSH is in range but you still feel tired, the dose may not be the cause. Tiredness has many causes, and some are common in people with thyroid disease. Tell your prescriber how you feel and ask whether low iron, low B12 or low vitamin D has been checked.
A worked example of reading the trend
- 1Baseline on the old doseTSH 8.0 mIU/L, measured at a routine check. The prescriber raises the dose.
- 2Early draw at week 2TSH 5.8 mIU/L. Still above range, but the draw came too soon to reflect the new dose. This reading alone says little about whether the new dose is right.
- 3Timed recheck at week 8TSH 2.6 mIU/L, inside the 4 to 8 week window 7. This value shows where the new dose has landed.
- 4Stable follow-upWith TSH in range and no changes, the next check usually comes at about 6 months, then yearly 1.
Selenium is often sold for thyroid health; what the evidence shows is on the selenium page. For the broader set of thyroid tests and conditions, see the thyroid topic hub. Two other widely prescribed drugs have their own lab scorecards: statins for LDL and metformin for HbA1c, kidney function and B12.
Link · American Thyroid AssociationHypothyroidism (underactive thyroid)Patient information from the ATA on hypothyroidism and its treatment with levothyroxine.thyroid.org
Citations
- Jonklaas J et al. Guidelines for the treatment of hypothyroidism: prepared by the American Thyroid Association Task Force on Thyroid Hormone Replacement. Thyroid. 2014;24:1670-1751
- US FDA. Update: The FDA warns that biotin may interfere with lab tests (Safety Communication, 2017, updated 2019)
- Alexander EK et al. 2017 Guidelines of the American Thyroid Association for the diagnosis and management of thyroid disease during pregnancy and the postpartum. Thyroid. 2017;27:315-389
- Centanni M et al. Thyroxine in goiter, Helicobacter pylori infection, and chronic gastritis. N Engl J Med. 2006;354:1787-1795
- Flynn RW et al. Serum thyroid-stimulating hormone concentration and morbidity from cardiovascular disease and fractures in patients on long-term thyroxine therapy. J Clin Endocrinol Metab. 2010;95:186-193
- Bach-Huynh TG et al. Timing of levothyroxine administration affects serum thyrotropin concentration. J Clin Endocrinol Metab. 2009;94:3905-3912
- Garber JR et al. Clinical practice guidelines for hypothyroidism in adults: cosponsored by the American Association of Clinical Endocrinologists and the American Thyroid Association. Thyroid. 2012;22:1200-1235
Educational context only, not medical advice or a diagnosis. Always discuss your results with a clinician.