The essentials

"TSH with reflex to FT4" (in Italy, "TSH reflex") means the lab measures TSH first and adds FT4, the unbound active form of thyroxine, automatically, on the same blood sample, only if TSH falls outside set limits. If TSH is within those limits, FT4 is not run and your report may say "not performed". That is usually reassuring, but the shortcut can miss pituitary disease and a few other situations.

How do I read a TSH with reflex to FT4 result?

A reflex test is a rule written into the order: the second test runs only if the first result meets a condition[3]. In this fictitious US report, TSH was above the cut-off, so FT4 was added.

Fictitious example of a lab report line (United States format)
TestResultReference rangeFlag
TSH w/Reflex to FT46.12 uIU/mL0.450-4.500 uIU/mLH
FT4 (Direct), reflex1.05 ng/dL0.82-1.77 ng/dL-
FT3, reflexNot performed--
  1. TSH line: always performed. 0.450 to 4.500 uIU/mL is both the Labcorp adult range and its reflex limits[3][11]. uIU/mL and mU/L are the same unit.
  2. Flag H: TSH is above the limit, so FT4 was added without a new draw.
  3. FT4, reflex: within the adult range of 0.82 to 1.77 ng/dL[12]. High TSH with normal FT4 is called subclinical (mild) hypothyroidism[1].
  4. "Not performed": not an error. The rule for that test was not met.

A reflex line, an H flag or a test marked "not performed"? BloodSense explains every line of your report in plain words and helps you prepare questions for your doctor.

Explain my thyroid report

What does "TSH reflex" mean and why do labs use it?

TSH is made by the pituitary gland and tells the thyroid how much hormone to produce. The American Thyroid Association calls it the best first test of thyroid function[5]. In a large study, about 97% of people with a TSH between 0.4 and 4 mU/L had a normal FT4[2].

The reflex strategy, also called "TSH-first" or "cascade" testing, is written into the UK national guideline: measure TSH alone in adults when pituitary disease is not suspected; if TSH is above the range, measure FT4 on the same sample; if it is below, measure FT4 and FT3[1]. One draw is enough. For what each hormone does, see our guide to the TSH blood test and the thyroid panel with FT4 and FT3.

What TSH levels trigger a reflex FT4?

There is no single rule. Each lab sets its own cut-offs and decides what comes next.

Examples of reflex rules (TSH in uIU/mL, equal to mU/L)
Lab or guidelineFT4 added when TSH isWhat may come nextSource
Labcorp (United States)below 0.450 or above 4.500FT3 after a low TSH, TPO antibodies after a high TSH, depending on FT4Labcorp test 330015[3]
Michigan Medicine laboratories (United States)below 0.30 or above 5.50FT3 depending on the FT4 resultMLabs[4]
NICE guideline (United Kingdom)outside the lab's reference rangeFT3 too when TSH is lowNICE NG145[1]

Some labs set cut-offs a little outside the TSH range: in 120,403 people in Australia, reflexing only below 0.2 or above 6.0 mU/L cut FT4 testing by 34%, and most missed results were marginal[2]. A TSH of 4.8 can trigger FT4 in one lab and not another. Your laboratory's limits apply.

Why does my report say FT4 "not performed"?

Because your TSH stayed between the lab's two cut-offs, so the rule stopped after the first test. That is the expected outcome for most people, not a lost sample.

The reverse also happens: FT4, and sometimes FT3, appear below a TSH you thought was ordered alone, as on many Italian "TSH reflex" reports. Each added test has its own line and range and may be billed separately[3]. If TSH is low and FT4 is normal, FT3 helps confirm or rule out an overactive thyroid[5]; see our FT3 test guide.

When can a TSH reflex test miss a thyroid problem?

The rule assumes TSH reacts normally to thyroid hormone. When that link is disturbed, TSH alone can mislead[8].

  • Common A recent change in levothyroxine dose. TSH and T4 generally reach a new steady state after about 6 weeks[10], so NICE advises not to repeat tests sooner than 6 weeks[1]. After a very high TSH, return to range can take up to 6 months[1].
  • Common Treatment for an overactive thyroid. Labcorp states its reflex test is not meant for people on ablative or suppressive thyroid therapy[3], and NICE advises measuring TSH, FT4 and FT3 for people taking antithyroid drugs[1], as in Graves' disease.
  • Common Acute illness. NICE advises against thyroid testing during an acute illness unless the thyroid is the suspected cause[1]. TSH can be low during the illness and briefly high during recovery[8].
  • Common Pregnancy. The pregnancy hormone hCG lowers TSH in the first trimester[8]. Labcorp suggests upper limits of 2.5 uIU/mL in the first trimester and 3.0 later[11]. The American Thyroid Association advises asking for both TSH and FT4 in pregnancy[5]. See our hCG blood test guide.
  • Possible Biotin supplements. High doses can make thyroid results falsely high or low[1].
  • Possible Amiodarone (a heart rhythm drug). In the first 3 months TSH often rises, then returns to baseline, and FT4 tends to rise, even with a healthy thyroid. About 15 to 20% of users develop a real thyroid problem[9].
  • Rare Central hypothyroidism, from pituitary or hypothalamic disease. FT4 is low, but TSH is low, normal or only slightly raised, so no reflex is triggered. The European Thyroid Association calls it a major false negative of the reflex TSH strategy; it affects about 1 in 16,000 to 1 in 100,000 people[7].
  • Rare TSH-secreting pituitary tumors and thyroid hormone resistance, where TSH can be normal while thyroid hormones are high[8][11].

Why might my doctor order FT4 even with a normal TSH?

NICE recommends measuring TSH and FT4 together when pituitary disease is suspected, and in children and young people[1]. The European guideline recommends both tests to look for central hypothyroidism[7], for example in people with known pituitary disease, which can also show up as an abnormal prolactin level. NICE also suggests it for adults who still have symptoms on levothyroxine[1]. The request then says "TSH and FT4", not "TSH reflex". Our FT4 blood test guide explains how to read it.

What can affect a TSH reflex result before the blood draw?

No fasting is needed for TSH or FT4 alone; ask whether other tests drawn at the same time require it[6]. Check this list:

  • Stop biotin beforehand if your provider agrees: 2 days for the American Thyroid Association[5], 72 hours for Labcorp[3].
  • Note the date of your last levothyroxine dose change.
  • Say if you are pregnant or planning a pregnancy.
  • Mention amiodarone, lithium, steroids or heparin.
  • Never stop a prescribed medicine on your own[6].

Practical tips are in our guide to what to expect at a blood draw.

When should I see a doctor about my TSH reflex result?

When to see a doctor

  • Emergency: call 911 for a racing or irregular heartbeat with chest pain, fainting or confusion, or for severe drowsiness with a very low body temperature.
  • Within 24 to 48 hours if TSH and FT4 are both flagged and you feel unwell, if you are pregnant with an abnormal TSH, or if you have a pituitary condition and new headaches or vision changes.
  • At your next appointment if only TSH is mildly high or low and FT4 is normal, or if FT4 was "not performed" but you still have symptoms.

Related conditions: underactive thyroid (hypothyroidism), overactive thyroid (hyperthyroidism) and Hashimoto's thyroiditis.

Questions to ask your doctor

  1. Which TSH limits does this lab use to add FT4?
  2. Was FT4 skipped, and do I need it anyway because of my history?
  3. When should I repeat the test after my dose change?
  4. Do I need thyroid antibodies, such as TPO, to find the cause?

Frequently asked questions

Is TSH reflex a different test from TSH?

No. TSH is measured the same way; the order simply includes the rule for adding FT4[1][3].

Do I need a second blood draw if FT4 is added?

No. It runs on the sample already collected[1].

Is TSH reflex suitable for monitoring levothyroxine?

In primary hypothyroidism, TSH is the main monitoring test[1]. If hypothyroidism comes from the pituitary, TSH is not reliable and FT4 guides treatment[7].

  1. National Institute for Health and Care Excellence (NICE). Thyroid disease: assessment and management. NICE guideline NG145. 2019, updated 2023. nice.org.uk (accessed November 5, 2026)
  2. Henze M, Brown SJ, Hadlow NC, Walsh JP. Rationalizing thyroid function testing: which TSH cutoffs are optimal for testing [...] T4? J Clin Endocrinol Metab. 2017;102(11):4235-4241. doi:10.1210/jc.2017-01322 (accessed November 5, 2026)
  3. Labcorp. TSH With Reflex to [...] T4 (test 330015). labcorp.com (accessed November 5, 2026)
  4. Michigan Medicine Laboratories (MLabs). TSH With Reflex FT4 and FT3 as Indicated. mlabs.umich.edu (accessed November 5, 2026)
  5. American Thyroid Association. Thyroid Function Tests. thyroid.org (accessed November 5, 2026)
  6. MedlinePlus. TSH (Thyroid-Stimulating Hormone) Test. US National Library of Medicine. medlineplus.gov (accessed November 5, 2026)
  7. Persani L, Brabant G, Dattani M, et al. 2018 European Thyroid Association (ETA) guidelines on the diagnosis and management of central hypothyroidism. Eur Thyroid J. 2018;7(5):225-237. doi:10.1159/000491388 (accessed November 5, 2026)
  8. Koulouri O, Moran C, Halsall D, Chatterjee K, Gurnell M. Pitfalls in the measurement and interpretation of thyroid function tests. Best Pract Res Clin Endocrinol Metab. 2013;27(6):745-762. doi:10.1016/j.beem.2013.10.003 (accessed November 5, 2026)
  9. Bartalena L, Bogazzi F, Chiovato L, et al. 2018 European Thyroid Association (ETA) guidelines for the management of amiodarone-associated thyroid dysfunction. Eur Thyroid J. 2018;7(2):55-66. doi:10.1159/000486957 (accessed November 5, 2026)
  10. Jonklaas J, Bianco AC, Bauer AJ, et al. Guidelines for the treatment of hypothyroidism: prepared by the American Thyroid Association task force on thyroid hormone replacement. Thyroid. 2014;24(12):1670-1751. doi:10.1089/thy.2014.0028 (accessed November 5, 2026)
  11. Labcorp. Thyroid-stimulating Hormone (TSH) (test 004259). labcorp.com (accessed November 5, 2026)
  12. Labcorp. Thyroxine (T4), [...], Direct, Serum (test 001974). labcorp.com (accessed November 5, 2026)

This page is general information. It does not replace the advice of a doctor, who interprets your results with your symptoms, history and examination.