What Are Optimal Thyroid Levels?

A Guide to Thyroid Function Testing

Optimal thyroid levels: A gloved hand holds a thyroid lab test sample

Key Takeaways:

  • TSH and free T4 are very accurate for diagnosis of hypothyroidism and hyperthyroidism.
  • Standard lab reference ranges are valid for diagnosing thyroid conditions.
  • Patients taking thyroid replacement hormone may benefit from targeting a narrower lab range.

If you’ve just had a blood test for a suspected thyroid condition, you’ll want to know if your thyroid hormone levels are optimal. But what are optimal thyroid levels?

Unfortunately, there is a lot of unnecessary debate and confusion on this topic.

Despite what many practitioners would have you believe, assessing thyroid hormone levels based on thyroid function tests is straightforward. A TSH test along with a free T4 test are very accurate for testing thyroid disorders, as shown in this chart.

 TSHFree T4
Hypothyroid Low levels of thyroid hormone500 F 103111477 XIiV6PZrXssnrr4MwD3gie0fAjgTayTy 4500 F 103111477 XIiV6PZrXssnrr4MwD3gie0fAjgTayTy 3
Subclinical Hypothyroid Mild hypothyroid that doesn’t usually need medication  500 F 103111477 XIiV6PZrXssnrr4MwD3gie0fAjgTayTy 4What Are Optimal Thyroid Levels? - 0fd156e4 c812 4974 9cd1 3b14df1c41cd
Central Hypothyroid* A rare disorder in the pituitary gland that causes hypothyroidWhat Are Optimal Thyroid Levels? - 0fd156e4 c812 4974 9cd1 3b14df1c41cd500 F 103111477 XIiV6PZrXssnrr4MwD3gie0fAjgTayTy 3
Hyperthyroid High levels of thyroid hormone500 F 103111477 XIiV6PZrXssnrr4MwD3gie0fAjgTayTy 3500 F 103111477 XIiV6PZrXssnrr4MwD3gie0fAjgTayTy 4

*  Technically TSH levels for central hypothyroid could be high, normal or low, but 84% of cases will have normal TSH [1]

It’s also important to know that standard lab values shown on your thyroid panel test are valid for diagnosing thyroid issues.

Here are the values that you’re most likely to see on your lab test results.

TSH (Thyroid Stimulating Hormone)

High TSH indicates hypothyroidism when paired with low free T4.. Low TSH suggests hyperthyroidism when paired with high free T4.

TSH is made by the pituitary gland and stimulates hormone production in the thyroid gland. If the pituitary gland senses that thyroid hormone levels in your body are low, it will produce more TSH in order to get the thyroid gland to make more hormones. Alternatively, if your pituitary gland senses that thyroid hormone levels in your body are high, it will produce less TSH.

  • The standard lab reference range for TSH is 0.45−4.5 IU/mL [2 Trusted SourcePubMedGo to source]
  • The standard lab range may require adjustments if you are pregnant or over 60 years of age
  • For hypothyroid patients who take thyroid hormone replacement, optimal levels of TSH are below 2.5 IU/mL [3]

It is very important that your practitioner does not apply the standard TSH lab range to patients taking thyroid medication like Synthroid. This is a common error in functional medicine that causes overdiagnosis of hypothyroidism and leads to unnecessary treatment.

Free T4

Low free T4 indicates hypothyroidism (underactive thyroid) when paired with high TSH. High free T4 indicates hyperthyroidism (overactive thyroid) when paired with low TSH.

T4, or thyroxine, is a thyroid hormone produced by the thyroid gland. T4 plays an important role in regulating your energy and metabolism. There are two kinds of T4 hormones:

  • Free T4: T4 hormone which is available for use by the body’s tissues
  • Bound T4: T4 hormone which is bound to a protein and can’t be used by the body’s tissues.

Lab tests can measure Free T4 or Total T4 (Free T4 + Bound T4). Free T4 is the more accurate test for thyroid function.

  • The standard lab reference range for free T4 is 0.82−1.77 ng/dL.
  • For hypothyroid patients who take thyroid hormone replacement, optimal T4 levels may be in the higher half of the lab range, however research to support this is lacking.

Free T3

T3, or triiodothyronine, is a thyroid hormone produced in small amounts by the thyroid gland. Free T3 is an active thyroid hormone. Reverse T3 may also be measured, however it is inactivated T3.

Free T3 may be included in a thyroid panel, but it’s not an accurate measurement of thyroid dysfunction. Patients with low Free T3 (and high reverse T3):

  • Are usually inflamed
  • May have digestive issues
  • May be eating too few calories or too few carbs
  • May be stressed or sleeping poorly

Lab ranges:

  • The standard lab reference range for free T3 is 2.0−4.4 pg/mL.
  • For hypothyroid patients who take thyroid hormone replacement, optimal free T3 levels may be above 3 pg/mL. However, research to support this is lacking.

Some practitioners prescribe T3 medication for patients with low free T3. However, T3 medication comes with risks. Combination therapy with T4 and T3 for hypothyroid patients should only be prescribed after trying other treatments.

A better and more natural approach focuses on the gut-thyroid connection, taking steps to improve gut health, modulate the immune system and decrease inflammation. This can be accomplished through following a better diet, taking quality probiotic supplements, reducing stress, and sleeping more. This will often lead to a resolution of the symptoms that were prompting evaluation of T3 to begin with, such as fatigue [4 Trusted SourcePubMedGo to source] or depression. [5 Trusted SourcePubMedGo to source]

Optimal thyroid levels: hands hold a hologram of the gut system

Thyroid Antibodies

Autoimmune disease is the most common cause of both hypothyroid (Hashimoto’s disease) and hyperthyroid (Grave’s disease). [6] Autoimmune thyroid disease is diagnosed by testing for thyroid antibodies: 

  • TPO(thyroperoxidase antibodies) levels provide the most reliable measure of thyroid autoimmunity. [7]
  • TG (thyroglobulin antibodies) levels may also be included in antibody testing but are not an accurate predictor of autoimmunity.[8]

TPOLab Range

  • TPO levels over 35 IU/mL are generally considered positive for thyroid antibodies.
    • However, positive lab results for thyroid antibodies do not mean you will become hypothyroid or hyperthyroid.
  • TPO levels between 35 and 500 IU/mL indicate minimal risk of progression to hypothyroid or hyperthyroid. [9 Trusted SourcePubMedGo to source]
  • TPO levels over 500 IU/mL indicate moderate risk of progression to hypothyroid. [10 Trusted SourcePubMedGo to source, 11]

TG Lab Range

  • TG levels over 0.9 IU/mL are generally considered positive for thyroid antibodies. However, this does not mean you will become hypothyroid or hyperthyroid.
  • TG levels over 9 IU/mL may indicate risk of progression to hypothyroid. However, research data is not conclusive. [12 Trusted SourcePubMedGo to source]

Elevated Thyroid Antibodies

Not all patients with elevated thyroid antibodies will become hypothyroid or hyperthyroid. In fact, the vast majority DO NOT.

At the beginning of a large thyroid study that followed 5,783 people for 9.1 years [13 Trusted SourcePubMedGo to source], the majority of those who were positive for TPO  antibodies had healthy thyroid function:

  • 10.9% were overt hypothyroid
  • 3.4% were hyperthyroid
  • 62.3% had normal thyroid hormone levels

Follow-up thyroid tests were done at year 3, 6, and 9. At each 3 year interval, TPO positive subjects had a 10-20% likelihood of becoming hypothyroid. The likelihood of becoming hyperthyroid was less than 3%.

Patients at highest risk of becoming hypothyroid have TPO levels greater than 500.

Other Tests for Thyroid Health

Optimal thyroid levels: A doctor with thyroid graphics

While thyroid lab tests are the most common way to check your thyroid health, additional tests may have value.

Thyroid Ultrasound

Thyroid ultrasound is used to visualize the anatomy of the thyroid gland. Thyroid ultrasound can identify thyroid nodules and, according to one study, may also predict those who may become hypothyroid or are less likely to be able to come off thyroid medication. [14 Trusted SourcePubMedGo to source]

Ferritin Test

If you take thyroid hormone replacement and still have persistent symptoms, a ferritin test may be helpful because 10 – 40% of hypothyroid patients have chronic low stomach acid, [15] which can lead to poor iron absorption.

A ferritin test measures the amount of iron stored in the blood. Hypothyroid patients may improve their thyroid symptoms by getting their ferritin levels up to 100 with iron supplementation:

  • In a small study of 25 hypothyroid women who were taking thyroid medication but still not feeling well, iron supplementation alleviated symptoms in roughly 75% of the patients. [16]

Can You Test Your Thyroid at Home?

Some practitioners suggest that you can monitor for an underactive thyroid problem at home by taking your temperature. While it’s true that hypothyroidism is associated with low body temperature, a large study of 35,488 patients found a difference of only .0234 degrees Fahrenheit between hypothyroid patients and subjects with normal thyroid function. [17 Trusted SourcePubMedGo to source] With such a small distinction in temperature, home temperature testing is not likely to be very accurate.

On the other hand, TSH and free T4 tests are very accurate and not expensive. If you suspect a thyroid disorder, please see a healthcare provider for one of these simple and accurate blood tests.

Overdiagnosis of Hypothyroid is Common

Research suggests that thyroid medication is overprescribed because many practitioners apply a very narrow range when reading thyroid hormone test results.

One very insightful study showed that up to 60% of patients may be taking thyroid hormone replacement unnecessarily. [18 Trusted SourcePubMedGo to source] In this study, 291 patients taking Levothyroxine (Synthroid) were asked to pause their medication for 6-8 weeks. Many of these patients had been taking thyroid medication for years but did not have strong diagnostic indicators of thyroid disease.

  • After going without synthetic thyroid hormone replacement for several weeks, patients were given thyroid function tests. 60.8% of patients had results in the normal range, meaning their body produces enough thyroid hormone and they did not require thyroid medication.
Optimal Thyroid Levels: An infographic showing the results of patients who paused thyroid medication for 6-8 weeks

While both conventional and functional medicine practitioners over diagnose thyroid disease, this is an area where conventional medicine tends to get it right more often than functional medicine.  

If you suspect your doctor rushed to diagnose you and hand over a pill, get a second opinion. If your lab results did not show abnormally high TSH paired with abnormally low free T4 (based on standard lab reference ranges) then you may not be truly hypothyroid and may not need medication. Of course, always check with your doctor before discontinuing your medication.

If you take thyroid medication, have normal lab results and you are still suffering from hypothyroid symptoms, the problem is likely not your thyroid. These symptoms can also result from gut dysbiosis, immune dysregulation, and inflammation. There are simple and practical steps you can take to improve your gut health and restore the gut-thyroid connection.

Read Amy’s story of resolving thyroid symptoms by addressing gut health.

➕ References
  1. Ross D, Cooper D, Mulder J. Central hypothyroidism. UpToDate, Inc. 2019. 
  2. Thayakaran R, Adderley NJ, Sainsbury C, et al. Thyroid replacement therapy, thyroid stimulating hormone concentrations, and long term health outcomes in patients with hypothyroidism: longitudinal study. BMJ. 2019;366:l4892. Published 2019 Sep 3. doi:10.1136/bmj.l4892 Trusted SourcePubMedGo to source
  3. American Thyroid Association. Optimal Thyroid Health for All. Q and A: TSH (thyroid stimulating hormone).
  4. Hustoft TN, Hausken T, Ystad SO, et al. Effects of varying dietary content of fermentable short-chain carbohydrates on symptoms, fecal microenvironment, and cytokine profiles in patients with irritable bowel syndrome. Neurogastroenterol Motil. 2017;29(4):10.1111/nmo.12969. doi:10.1111/nmo.12969 Trusted SourcePubMedGo to source
  5. Ng QX, Peters C, Ho CYX, Lim DY, Yeo WS. A meta-analysis of the use of probiotics to alleviate depressive symptoms. J Affect Disord. 2018;228:13-19. doi:10.1016/j.jad.2017.11.063 Trusted SourcePubMedGo to source
  6. Fröhlich E, Wahl R. Thyroid Autoimmunity: Role of Anti-thyroid Antibodies in Thyroid and Extra-Thyroidal Diseases. Front Immunol. 2017;8:521. Published 2017 May 9. doi:10.3389/fimmu.2017.00521
  7. Fröhlich E, Wahl R. Thyroid Autoimmunity: Role of Anti-thyroid Antibodies in Thyroid and Extra-Thyroidal Diseases. Front Immunol. 2017;8:521. Published 2017 May 9. doi:10.3389/fimmu.2017.00521
  8. Fröhlich E, Wahl R. Thyroid Autoimmunity: Role of Anti-thyroid Antibodies in Thyroid and Extra-Thyroidal Diseases. Front Immunol. 2017;8:521. Published 2017 May 9. doi:10.3389/fimmu.2017.00521
  9. Ehlers M, Jordan AL, Feldkamp J, et al. Anti-Thyroperoxidase Antibody Levels >500 IU/ml Indicate a Moderately Increased Risk for Developing Hypothyroidism in Autoimmune Thyroiditis. Horm Metab Res. 2016;48(10):623-629. doi:10.1055/s-0042-112815 Trusted SourcePubMedGo to source
  10. Bohuslavizki KH, vom Baur E, Weger B, et al. Evaluation of chemiluminescence immunoassays for detecting thyroglobulin (Tg) and thyroid peroxidase (TPO) autoantibodies using the IMMULITE 2000 system. Clin Lab. 2000;46(1-2):23-31. Trusted SourcePubMedGo to source
  11. https://www.hindawi.com/journals/ad/2019/1684074/
  12. Siriwardhane T, Krishna K, Ranganathan V, et al. Significance of Anti-TPO as an Early Predictive Marker in Thyroid Disease. Autoimmune Dis. 2019;2019:1684074. Published 2019 Jul 28. doi:10.1155/2019/1684074 Trusted SourcePubMedGo to source
  13. Amouzegar A, Gharibzadeh S, Kazemian E, Mehran L, Tohidi M, Azizi F. The Prevalence, Incidence and Natural Course of Positive Antithyroperoxidase Antibodies in a Population-Based Study: Tehran Thyroid Study. PLoS One. 2017;12(1):e0169283. Published 2017 Jan 4. doi:10.1371/journal.pone.0169283 Trusted SourcePubMedGo to source
  14. Livadas S, Bothou C, Androulakis I, Boniakos A, Angelopoulos N, Duntas L. Levothyroxine Replacement Therapy and Overuse: A Timely Diagnostic Approach [published online ahead of print, 2018 Nov 30]. Thyroid. 2018;10.1089/thy.2018.0014. doi:10.1089/thy.2018.0014 Trusted SourcePubMedGo to source
  15. Cellini M, Santaguida MG, Virili C, et al. Hashimoto’s Thyroiditis and Autoimmune Gastritis. Front Endocrinol (Lausanne). 2017;8:92. Published 2017 Apr 26. doi:10.3389/fendo.2017.00092
  16. Worcester, S. Iron deficiency may explain persistent hypothyroidism symptoms. MDedge Endocrinology. 2019.
  17. Obermeyer Z, Samra JK, Mullainathan S. Individual differences in normal body temperature: longitudinal big data analysis of patient records. BMJ. 2017;359:j5468. Published 2017 Dec 13. doi:10.1136/bmj.j5468 Trusted SourcePubMedGo to source
  18. Livadas S, Bothou C, Androulakis I, Boniakos A, Angelopoulos N, Duntas L. Levothyroxine Replacement Therapy and Overuse: A Timely Diagnostic Approach [published online ahead of print, 2018 Nov 30]. Thyroid. 2018;10.1089/thy.2018.0014. doi:10.1089/thy.2018.0014 Trusted SourcePubMedGo to source

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