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Pacific · Reverse T3 Test – rT3 Testing

Reverse T3 Test – rT3 Testing in California

This page explains the reverse t3 test – rt3 testing, what it measures, and how to order it. The Reverse T3 blood test measures a form of thyroid hormone that can help clarify unusual thyroid function test results. It is often used alongside other thyroid tests to give a fuller picture of how the body is handling thyroid hormone.

Sample typeBlood and/or urine at a collection lab
ReferralIncluded — no referral from your own doctor needed

Lab requirements in California

Here is what you need before ordering and completing the Reverse T3 Test – rT3 Testing in CA. Review each item and confirm the details with the collection lab.

Physician's order
Included. A physician's order is required for lab testing and is provided for you by our partner's physicians — no referral from your own doctor is needed.
Fasting
Some tests require fasting or avoiding certain foods or drinks beforehand. You will be informed when you order — confirm the details with the collection lab.
Preparation
Preparation instructions can vary depending on the laboratory and the other tests being performed at the same time. Always follow the specific instructions provided by the collection lab or…
Sample & visit
A blood and/or urine sample is collected; the visit usually takes about 10 minutes.
Identification
Bring the lab requisition form from your account. The collection lab may also ask for a government-issued photo ID.
Turnaround
1-3 business days
Availability
Our partner lab does not serve ND, RI, NY, NJ, MD or HI. Confirm availability for your location with the collection lab.

Before you go

  • Order your test before visiting a lab, then use the requisition form in your account.
  • Print the requisition form, or fax it to the lab directly from your account.
  • No scheduling is needed — visit the collection lab during its normal business hours.
  • A blood and/or urine sample is collected; the visit usually takes about 10 minutes.
  • Bring a government-issued photo ID in case the lab requests it.
  • Results are posted to your secure account within 1–3 business days.

Requirements are summarized from our partner lab's published policies. Preparation and availability can change — confirm current instructions with the collection lab before your visit.

What this test is

Reverse T3 is a biologically inactive form of thyroid hormone produced when the body converts thyroxine (T4) in an alternative pathway. This test measures the amount of reverse T3 circulating in the blood. It is not a stand-alone diagnostic test but is interpreted with other thyroid hormone measurements.

What it measures

  • Reverse T3 — reflects the amount of this inactive thyroid hormone form in the blood
  • Total T3 — often measured alongside to assess active thyroid hormone levels
  • Free T4 — helps evaluate the main thyroid hormone produced by the thyroid gland
  • TSH — indicates how the pituitary gland is responding to thyroid hormone levels

What the test involves

A healthcare professional collects a small sample of blood, usually from a vein in your arm, using a standard blood draw. The sample is then sent to a laboratory for analysis.

Why it is often ordered

A clinician may order a reverse T3 test when thyroid test results are confusing or when a nonthyroidal illness is suspected. It is typically used to help evaluate whether symptoms or abnormal thyroid tests might be related to how the body is converting thyroid hormones.

Preparation

Preparation instructions can vary depending on the laboratory and the other tests being performed at the same time. Always follow the specific instructions provided by the collection lab or your healthcare provider.

How results are reported

Results are reported as a numeric value for reverse T3, and the laboratory will provide a reference range for comparison. Reference ranges can differ between laboratories because of differences in equipment, methods, and population, so your result should be interpreted using the range from the lab that performed the test.

Before you go

  • Whether any other tests will be drawn at the same time and if fasting is required.
  • The specific reference range used by the laboratory performing the test.
  • How and when you will receive your results.

Cities

Reverse T3 Test – rT3 Testing in California communities

Choose your city for local context and to order this test.

Los Angeles12,121,244Reverse T3 Test – rT3 TestingSan Francisco3,290,197Reverse T3 Test – rT3 TestingSan Diego3,084,174Reverse T3 Test – rT3 TestingRiverside2,022,285Reverse T3 Test – rT3 TestingSacramento1,924,167Reverse T3 Test – rT3 TestingSan Jose1,729,879Reverse T3 Test – rT3 TestingFresno719,558Reverse T3 Test – rT3 TestingConcord640,270Reverse T3 Test – rT3 TestingBakersfield590,845Reverse T3 Test – rT3 TestingMission Viejo588,540Reverse T3 Test – rT3 TestingMurrieta509,526Reverse T3 Test – rT3 TestingLong Beach466,565Reverse T3 Test – rT3 TestingOakland437,548Reverse T3 Test – rT3 TestingStockton406,822Reverse T3 Test – rT3 TestingVictorville389,060Reverse T3 Test – rT3 TestingModesto382,994Reverse T3 Test – rT3 TestingLancaster381,732Reverse T3 Test – rT3 TestingOxnard366,330Reverse T3 Test – rT3 TestingIndio361,831Reverse T3 Test – rT3 TestingAnaheim348,204Reverse T3 Test – rT3 TestingAntioch322,796Reverse T3 Test – rT3 TestingSanta Rosa314,196Reverse T3 Test – rT3 TestingSanta Ana313,818Reverse T3 Test – rT3 TestingIrvine297,868Reverse T3 Test – rT3 TestingSanta Clarita275,230Reverse T3 Test – rT3 TestingChula Vista274,818Reverse T3 Test – rT3 TestingVisalia249,804Reverse T3 Test – rT3 TestingFremont231,502Reverse T3 Test – rT3 TestingSan Bernardino220,821Reverse T3 Test – rT3 TestingThousand Oaks211,045Reverse T3 Test – rT3 TestingMoreno Valley208,371Reverse T3 Test – rT3 TestingFontana208,087Reverse T3 Test – rT3 TestingSalinas199,290Reverse T3 Test – rT3 TestingHuntington Beach198,735Reverse T3 Test – rT3 TestingSanta Barbara198,240Reverse T3 Test – rT3 TestingGlendale196,512Reverse T3 Test – rT3 TestingHemet191,170Reverse T3 Test – rT3 TestingVallejo181,882Reverse T3 Test – rT3 TestingElk Grove175,510Reverse T3 Test – rT3 TestingOntario175,223Reverse T3 Test – rT3 TestingOceanside174,461Reverse T3 Test – rT3 TestingRancho Cucamonga173,946Reverse T3 Test – rT3 TestingGarden Grove172,708Reverse T3 Test – rT3 TestingPalmdale167,987Reverse T3 Test – rT3 TestingSanta Cruz165,933Reverse T3 Test – rT3 TestingHayward162,254Reverse T3 Test – rT3 TestingCorona157,844Reverse T3 Test – rT3 TestingMerced157,617Reverse T3 Test – rT3 TestingSunnyvale155,550Reverse T3 Test – rT3 TestingPomona151,592Reverse T3 Test – rT3 TestingEscondido151,443Reverse T3 Test – rT3 TestingTorrance147,156Reverse T3 Test – rT3 TestingRoseville145,687Reverse T3 Test – rT3 TestingFairfield145,163Reverse T3 Test – rT3 TestingFullerton142,964Reverse T3 Test – rT3 TestingSanta Maria142,230Reverse T3 Test – rT3 TestingOrange139,195Reverse T3 Test – rT3 TestingPasadena138,771Reverse T3 Test – rT3 TestingYuba City129,536Reverse T3 Test – rT3 TestingSanta Clara127,922Reverse T3 Test – rT3 Testing