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What Does Your Thyroid Function Test Really Tell You?

Written by: Dr. Diana Rangaves, PharmD

Thyroid function tests assess how well your thyroid gland is working by measuring TSH, T3, and T4 levels. These tests are typically done at healthcare centers to help detect hypothyroidism, hyperthyroidism, or normal thyroid function.

Most people get tested due to symptoms like weight changes, fatigue, hair thinning, or irregular heart rate, while others do it for routine checkups or due to family history.1

However, simply taking the test isn't enough. You need to understand your results to take the right steps for your health. If you've recently had a thyroid test and are unsure what it means, this article will explain key markers, their implications, and what to do next.

Understanding Thyroid Function Markers

When you take a thyroid function test, the results typically include three primary markers: TSH (thyroid-stimulating hormone), T3 (triiodothyronine), and T4 (thyroxine). Each plays a role in how your thyroid functions. Understanding what they mean can help you interpret your test results accurately:

TSH (Thyroid-Stimulating Hormone)

The pituitary gland produces TSH and regulates thyroid hormone production. It signals the thyroid gland to release T3 and T4.2, 3, 4, 5

Normal Range: Typically between 0.5 – 5.0 mIU/L, but this may vary depending on the lab.
High TSH: May indicate hypothyroidism (underactive thyroid), meaning your thyroid isn't producing enough hormones.
Low TSH: May suggest hyperthyroidism (overactive thyroid), where your thyroid is making too many hormones.

T3 (Triiodothyronine)

T3 is one of the thyroid hormones that directly influence metabolism, energy levels, and heart rate.6 It exists in two forms: Total T3, which includes both bound and free T3, and Free T3 (FT3), the active form that is readily available for use by the body.7, 8

Normal Range: Around 2.3 – 4.2 pg/mL (for free T3) and around 80–180 ng/dL (for total T3).
High T3 Levels: Often linked to hyperthyroidism, causing symptoms like rapid heartbeat, anxiety, and weight loss.

T4 (Thyroxine)

T4 is the storage form of the thyroid hormone and is converted into T3 when needed. Like T3, it also has two forms: Total T4, which measures both bound and free T4, and Free T4 (FT4), the active form that affects metabolism.9, 10

Normal Range: Typically 0.8 – 1.8 ng/dL (for free T4).
High T4 Levels: Indicate hyperthyroidism, where the thyroid is overproducing hormones.
Low T4 Levels: Suggest hypothyroidism, meaning the thyroid isn't producing enough hormones.

Reverse T3 (rT3)

Reverse T3 is an inactive form of T3 produced during stress or illness. High levels may indicate that your body is converting T4 into rT3 instead of active T3, which can lead to hypothyroid symptoms even if TSH and T4 levels appear normal.11

Interpreting Your Thyroid Test Results

Once you receive your thyroid function test results, understanding what they mean is essential for determining your next steps.

  • High TSH with Low T3 and T4: This pattern usually indicates primary hypothyroidism, meaning the thyroid gland isn't producing enough hormones.3
  • Low TSH with High T3 and T4: This is a common sign of primary hyperthyroidism.3
  • High TSH with Normal T3 and T4: This may indicate subclinical hypothyroidism.3
  • Low TSH with Normal T3 and T4: This pattern suggests subclinical hyperthyroidism.3
  • Low T3 with Normal T4 and TSH: This condition, sometimes called low T3 syndrome or euthyroid sick syndrome, can occur during periods of severe illness, stress, or malnutrition.12

Factors That May Affect Your Test Results

Thyroid function tests provide important insights, but several factors can influence the accuracy of your results.

  • Medications and Supplements: Certain medications can interfere with thyroid hormone levels.13
  • Recent Illness or Stress: Severe illness or chronic stress can temporarily lower T3 levels.12
  • Time of Day and Fasting: TSH levels fluctuate throughout the day and fasting can impact thyroid levels.14, 15
  • Pregnancy and Menstrual Cycle: Pregnancy and hormonal changes during the menstrual cycle can influence thyroid function.16,17
  • Lab Variations and Testing Errors: Different labs use different reference ranges; retesting may be necessary if results seem inconsistent.

Next Steps After Receiving Your Results

  • If Your Results Are Normal: No action may be needed, but follow up on persistent symptoms.
  • If Your Results Indicate Hypothyroidism: Treatment usually includes levothyroxine.18
  • If Your Results Indicate Hyperthyroidism: Treatment may involve anti-thyroid medication or beta-blockers.19
  • If Your Results Suggest Subclinical Thyroid Dysfunction: Regular monitoring is important.
  • If Your Results Are Inconclusive or Unexpected: Retesting may be needed to confirm accuracy.

Take Control of Your Thyroid Health with Everlywell

Taking charge of your thyroid health starts with understanding your test results. Whether your levels are normal, borderline, or indicate a thyroid disorder, the next step is making informed decisions about your health.

With Everlywell's at-home Thyroid Test, you can check your TSH, T3, and T4 levels with just a simple finger prick. Your results are reviewed by a certified lab and come with easy-to-understand insights to help you take the right next steps.

Don't leave your thyroid health to guesswork. Order your Everlywell Thyroid Test today.

References
  1. Symptoms. nhs.uk. Published April 18, 2023. Accessed February 21, 2025. https://www.nhs.uk/conditions/underactive-thyroid-hypothyroidism/symptoms/
  2. Pirahanchi Y, Toro F, Jialal I. Physiology, Thyroid Stimulating Hormone [Internet]. Updated 2023 May 1. In: StatPearls. Treasure Island (FL): StatPearls Publishing. Accessed February 21, 2025. https://www.ncbi.nlm.nih.gov/books/NBK537039/
  3. Normal thyroid hormone levels. UCLA Health. Accessed February 21, 2025. https://www.uclahealth.org/medical-services/endocrinology/thyroid/normal-thyroid-hormone-levels
  4. Underactive thyroid (hypothyroidism): Learn More. InformedHealth.org. Updated 2024 May 28. Accessed February 21, 2025. https://www.ncbi.nlm.nih.gov/books/NBK279539/
  5. Hyperthyroidism (Overactive thyroid). NIDDK. Published January 16, 2025. Accessed February 21, 2025. https://www.niddk.nih.gov/health-information/endocrine-diseases/hyperthyroidism
  6. Thyroid gland. Healthdirect Australia. Accessed February 21, 2025. https://www.healthdirect.gov.au/thyroid-gland
  7. Triiodothyronine (T3) tests. Accessed February 21, 2025. https://medlineplus.gov/lab-tests/triiodothyronine-t3-test/
  8. T3 test. Testing.com. Published November 29, 2022. Accessed February 21, 2025. https://www.testing.com/tests/t3/
  9. Pediatric Thyroid Function Tests. American Thyroid Association. Published June 8, 2020. Accessed February 21, 2025. https://www.thyroid.org/pediatric-thyroid-function-tests/
  10. T4 test. UCSF Health. Published August 6, 2024. Accessed February 21, 2025. https://www.ucsfhealth.org/medical-tests/t4-test
  11. What is the Reverse T3 Syndrome? Alan R. Jacobs MD PC. Published February 12, 2022. Accessed February 21, 2025. https://alanjacobsmd.com/what-is-reverse-t3-syndrome/
  12. Ganesan K, Anastasopoulou C, Wadud K. Euthyroid Sick Syndrome [Internet]. Updated 2022 Dec 8. In: StatPearls. Accessed February 21, 2025. https://www.ncbi.nlm.nih.gov/books/NBK507815/
  13. Dong BJ. How medications affect thyroid function. West J Med. 2000 Feb;172(2):102-6. doi: 10.1136/ewjm.172.2.102. PMID: 10693372; PMCID: PMC1070767.
  14. Time of day makes a difference in thyroid testing. Emerald City Naturopathic Clinic. Published August 22, 2012. Accessed February 21, 2025. https://emeraldcityclinic.com/time-of-day-makes-a-difference-in-thyroid-testing/
  15. Nair R, et al. Does fasting or postprandial state affect thyroid function testing? Indian J Endocrinol Metab. 2014 Sep;18(5):705-7. doi:10.4103/2230-8210.139237. PMID: 25285290; PMCID: PMC4171896.
  16. Singh S, et al. Thyroid Disease and Pregnancy [Internet]. Updated 2025 Jan 19. In: StatPearls. Accessed February 21, 2025. https://www.ncbi.nlm.nih.gov/books/NBK279116/
  17. Jacobson MH, et al. Thyroid hormones and menstrual cycle function. Paediatr Perinat Epidemiol. 2018 May;32(3):225-234. doi:10.1111/ppe.12462. PMID: 29517803; PMCID: PMC5980701.
  18. Levothyroxine. nhs.uk. Published October 28, 2024. Accessed February 21, 2025. https://www.nhs.uk/medicines/levothyroxine/
  19. Hyperthyroidism and thyrotoxicosis medication: antithyroid agents, beta blockers. Accessed February 21, 2025. https://www.ncbi.nlm.nih.gov/books/NBK279005/
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