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What Are Optimal Thyroid Levels?

A Guide to Thyroid Function Testing

Key Takeaways:
  • A standard thyroid panel that measures TSH and free T4  hormones can accurately diagnose hypothyroidism but needs to be interpreted with nuance. 
  • Subclinical hypothyroidism may be overdiagnosed in some patient groups if practitioners apply ‘normal range’ levels of TSH too strictly when interpreting results.
  • Medication isn’t always necessary for mild cases of hypothyroid; working on your gut health can often turn things around.

If you’ve just had a blood test for a suspected thyroid condition, you’ll want to know whether your thyroid hormone levels are in the right range. 

But what are optimal thyroid levels? Unfortunately, there is a lot of unnecessary debate and confusion on this topic.

However, interpreting basic thyroid function tests, i.e. those that measure your levels of thyroid-stimulating hormone (TSH) and thyroxine hormone (aka T4), is actually relatively straightforward. There are a few caveats, but I’ll walk you through them later in this article. 

Diagnosing Hypo-and Hyperthyroid From TSH and T4 Levels

Before broadening the topic to look at other potential measures of thyroid health, let’s look at the two commonest lab values on your thyroid panel test. As I mentioned, these are TSH and T4.

For adults who aren’t pregnant, and are under 60:

  • The healthy range of TSH is about 0.45−4.5 IU/mL [1]. 
  • The healthy range of free T4 is 0.82−1.76 ng/dL [2].

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

Optimal Thyroid Levels

TSH and T4 Work on a Feedback Mechanism

The pituitary gland makes TSH and stimulates the production of thyroid hormones (mainly T4) by the thyroid gland.

If the pituitary gland senses that you have low thyroid hormone levels, it will produce more TSH to get the thyroid gland to make more thyroid hormones. As T4 levels get higher, your pituitary gland then begins to shut off TSH production until your levels T4 begin to fall again. 

When it’s working perfectly, this feedback loop constantly cycles to keep your thyroid hormones at optimal levels.  

However, it’s important that your practitioner doesn’t apply the standard TSH lab range in every case. The standard cutoffs don’t apply to patients taking the thyroid medication levothyroxine (brand name Synthroid). They also don’t apply in pregnancy. Additionally, they should be interpreted differently in older people — we’ll go into the specifics of how age affects your TSH levels a little later.

Optimal Thyroid Levels

What Is Clinical Hypothyroidism?

If your free T4 is below the normal range (less than 0.82 ng/dL and your TSH is above its normal range (greater than 0.176 ng/dL), this indicates clinical hypothyroidism

Symptoms of hypothyroidism include fatigue, weight gain, cognitive issues (brain fog), dry skin, hair loss, and feeling cold.

You can correct clinical hypothyroidism with thyroid hormone replacement therapy (usually levothyroxine) and improve the medication’s effects with healthy lifestyle changes.

What Is Subclinical Hypothyroidism?

In contrast, high TSH with normal free T4 suggests subclinical hypothyroidism.

You probably do not need thyroid hormones to treat this. You can likely treat it by tackling your gut health and the autoimmune issues that often underlie subclinical hypothyroidism.

What Is Hyperthyroidism?

If you have low levels of TSH, and high levels of T4 this suggests you are hyperthyroid, i.e. have an overactive thyroid. Hyperthyroidism is relatively uncommon and affects roughly 1.2% of people in the U.S. [3]. 

Graves’ disease, is an autoimmune condition that sometimes leads to hyperthyroidism and affects one in 200 people, the majority of whom are women [3, 4]. In Graves’ disease, you may have a low TSH and normal T4 level.

Symptoms of hyperthyroidism include weight loss, rapid heartbeat, irritability, fatigue, shakiness, frequent bowel movements, and an enlargement in the neck, called a goiter [5].

Central Hypothyroidism

There’s also a rare disorder of the pituitary gland that causes a hypothyroid condition called central hypothyroidism. In this case, you will usually have normal TSH levels, but T4 levels are low. 

What About T3?

Some thyroid panels may also include your level of another thyroid hormone, — T3, or triiodothyronine. This thyroid hormone is produced in smaller amounts than T4, as the body can make its own T3 from T4). Free T3 is the active thyroid hormone but reverse T3 (inactivated T3) may also be measured.

The standard lab reference range for free T3 is 2.0−4.4 pg/mL 

Low free T3 (or high reverse T3) can be a byproduct of poor nutrition, gut inflammation, and chronic illness [6]. Rather than taking a hormone replacement for low T3 levels, you’ll likely be better off treating gut imbalances and nutritional deficiencies.

That said, some practitioners do prescribe T3 along with T4 for hypothyroidism. Based on my clinical findings with patients over the years,  and the fact that T3 medication comes with potential cardiovascular risks [7], I usually only recommend trying combination T4 and T3 therapy  if T4 alone hasn’t worked well.

A better and more natural approach if you have low T3 is to focus on strengthening the gut-thyroid connection, i.e. by 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 better. This will often lead to a resolution of the symptoms, e.g. fatigue and low mood, that were prompting most patients to consider T3 to begin with.

Thyroid Antibody Testing

Autoimmune disease is the most common cause of both hypothyroidism and hyperthyroidism.  In both cases, an overactive immune system attacks the thyroid gland and unbalances thyroid hormones. 

Hashimoto’s thyroiditis is the autoimmune condition that can lead to hypothyroidism, and Grave’s disease is the autoimmune condition that can lead to hyperthyroidism  [8]. 

Autoimmune thyroid disease is diagnosed by testing for thyroid antibodies. However, it’s important to interpret these results with care. I’ll go into this more later, after we’ve looked briefly at types of antibody testing.

TPO Testing

TPO (thyroperoxidase antibodies) levels provide the most reliable measure of thyroid autoimmunity [9].

  • TPO levels over 35 IU/mL are generally considered positive for thyroid antibodies.
  • TPO levels between 35 and 500 IU/mL indicate minimal risk of progression to hypothyroid or hyperthyroid [10].
  • TPO levels over 500 IU/mL indicate a moderate risk of progression to hypothyroid [11, 12].

TG Testing

TG (thyroglobulin antibodies) levels may also be included in antibody testing but are not an accurate predictor of autoimmunity [12].

  • TG levels over 0.9 IU/mL are generally considered positive for thyroid antibodies. 
  • TG levels over 9 IU/mL may indicate a risk of progression to hypothyroid. However, research data is not conclusive.

Interpreting Elevated Thyroid Antibodies

A positive lab result for thyroid antibodies does not mean you will become hypothyroid or hyperthyroid. I can not stress this strongly enough because patients often worry unnecessarily.

Some stats from a thyroid study that followed 5,783 people for 9 years [13] are reassuring here.

At the beginning of the study, the majority of those who were positive for TPO antibodies had healthy thyroid function. However:

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

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

This means it’s not predetermined that you’ll develop a clinical thyroid condition when you have thyroid antibodies. You may be able to make lasting dietary and lifestyle changes that can reverse your autoimmune condition.

Overdiagnosis of Hypothyroid is Common

As I already briefly mentioned, research suggests that thyroid medication is overprescribed because many practitioners are too rigid in their interpretation of thyroid hormone test results.

One very insightful study showed that up to 60% of patients may be taking thyroid hormone replacement unnecessarily [14]. 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, all the patients then took thyroid function tests.

 60.8% of patients had results in the normal range, meaning their body was producing enough thyroid hormone, and they did not require thyroid medication.

You’re More Likely to Be Misdiagnosed If you Are Older

You’re more likely to be misdiagnosed with hypothyroidism if you are older because TSH values naturally rise with age but sometimes clinics do not adjust for this [15]. The standard lab ranges don’t apply if you are over 60 years old. 

This is illustrated well by a 2021 study of over 2,500 older people. It found that the estimated prevalence of subclinical hypothyroidism dropped hugely (to just 10% of the original estimate) when clinicians used an age-adjusted TSH reference range instead of the standard range detailed above [16]. 

A good rule of thumb from the accomplished thyroid expert Dr. Antonio Bianco is that for each decade of your older life, you can essentially go up a value of TSH. So,

  • If you are in your 60s, it’s okay to have TSH in the 6s. 
  • In your 70s, it’s okay to have TSH in the 7s.
  • In your 80s, it’s okay to have TSH in the 8s, and so on.

The takeway here is that it’s worth checking what cutoffs your lab used if you were diagnosed with either clinical or subclinical hypothyroidism as an older person. This is particularly the case if you have been told you need medications but don’t have significant symptoms.

Get a Second Opinion

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

If you suspect your diagnosis and/or treatment by a functional practitioner may have been premature or unnecessary, it pays to 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 probably are not truly hypothyroid
  • Or, if you take thyroid medication, have normal lab results, and are still suffering from “hypothyroid” symptoms, the problem is again not likely not be your thyroid. 

Symptoms that appear to be from your thyroid 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

Of course, if you have already started medication, always check with your doctor before discontinuing your medication.

Other Tests for Thyroid Health

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

Thyroid Ultrasound

A 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].

Ferritin Test

If you take thyroid hormone replacement and still have persistent symptoms, a ferritin test may be helpful. This is because 10–40% of hypothyroid patients have chronic low stomach acid [17], which can lead to poor iron absorption.

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

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 lower body temperature, a large study of 35,488 patients found a difference of only 0.0234 degrees Fahrenheit between hypothyroid patients and subjects with normal thyroid function [18]. 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, your best course of action is to see a healthcare provider for one of these simple and accurate blood tests.

Understanding Your Thyroid Results Is Empowering

Having your TSH and T4 levels tested should give you an accurate idea of whether you have a clinical thyroid problem. However,  care is needed in interpreting results that aren’t clear-cut, as you want to avoid medication that isn’t necessary. Specifically, age and symptomatology should be taken into account — especially if only your TSH level is out of  “normal” limits. 

If you have subclinical hypothyroidism, your best option is likely to take care of your underlying gut health. My thyroid course will show you how to self-manage thyroid conditions, including diets and supplements that help. You can also reach out for a virtual one-to-one consultation with one of our highly experienced practitioners.

The Ruscio Institute has developed a range of high-quality formulations to help our patients and audience. If you’re interested in learning more about these products, please click here. Note that there are many other options available, and we encourage you to research which products may be right for you.

➕ References

  1. Thayakaran R, Adderley NJ, Sainsbury C, Torlinska B, Boelaert K, Šumilo D, et al. Thyroid replacement therapy, thyroid stimulating hormone concentrations, and long term health outcomes in patients with hypothyroidism: longitudinal study. BMJ. 2019 Sep 3;366:l4892. DOI: 10.1136/bmj.l4892. PMID: 31481394. PMCID: PMC6719286.
  2. Lee J, Ha J, Jo K, Lim D-J, Lee J-M, Chang S-A, et al. High Normal Range of Free Thyroxine is Associated with Decreased Triglycerides and with Increased High-Density Lipoprotein Cholesterol Based on Population Representative Data. J Clin Med. 2019 May 28;8(6). DOI: 10.3390/jcm8060758. PMID: 31142048. PMCID: PMC6616420.
  3. Pokhrel B, Bhusal K. Graves Disease. In: StatPearls. Treasure Island (FL): StatPearls Publishing; 2021. PMID: 28846288.
  4. Vanderpump MPJ. The epidemiology of thyroid disease. Br Med Bull. 2011;99:39–51. DOI: 10.1093/bmb/ldr030. PMID: 21893493.
  5. De Leo S, Lee SY, Braverman LE. Hyperthyroidism. Lancet. 2016 Aug 27;388(10047):906–18. DOI: 10.1016/S0140-6736(16)00278-6. PMID: 27038492. PMCID: PMC5014602.
  6. 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 Dec;27(6):745–62. DOI: 10.1016/j.beem.2013.10.003. PMID: 24275187. PMCID: PMC3857600.
  7. Are There Long-Term Adverse Effects of T3 Therapy for Hypothyroidism? We All Want to Know! Clinical Thyroidology.
  8. Thyroid disease and autoimmune diseases – Autoimmunity – NCBI Bookshelf [Internet]. [cited 2021 Jul 1]. Available from:
  9. Fröhlich E, Wahl R. Thyroid Autoimmunity: Role of Anti-thyroid Antibodies in Thyroid and Extra-Thyroidal Diseases. Front Immunol. 2017 May 9;8:521. DOI: 10.3389/fimmu.2017.00521. PMID: 28536577. PMCID: PMC5422478.
  10. Ehlers M, Jordan AL, Feldkamp J, Fritzen R, Quadbeck B, Haase M, et al. Anti-Thyroperoxidase Antibody Levels >500 IU/ml Indicate a Moderately Increased Risk for Developing Hypothyroidism in Autoimmune Thyroiditis. Horm Metab Res. 2016 Sep 8;48(10):623–9. DOI: 10.1055/s-0042-112815. PMID: 27607246.
  11. Bohuslavizki KH, vom Baur E, Weger B, Krebs C, Saller B, Wetlitzky O, 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. PMID: 10745978.
  12. Siriwardhane T, Krishna K, Ranganathan V, Jayaraman V, Wang T, Bei K, et al. Significance of Anti-TPO as an Early Predictive Marker in Thyroid Disease. Autoimmune Dis. 2019 Jul 28;2019:1684074. DOI: 10.1155/2019/1684074. PMID: 31467701. PMCID: PMC6699358.
  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 Jan 4;12(1):e0169283. DOI: 10.1371/journal.pone.0169283. PMID: 28052092. PMCID: PMC5215694.
  14. Livadas S, Bothou C, Androulakis I, Boniakos A, Angelopoulos N, Duntas L. Levothyroxine replacement therapy and overuse: A timely diagnostic approach. Thyroid. 2018 Nov 30; DOI: 10.1089/thy.2018.0014. PMID: 30351232.
  15. Fu J, Wang Y, Liu Y, Song Q, Cao J, Peichang W. Reference intervals for thyroid hormones for the elderly population and their influence on the diagnosis of subclinical hypothyroidism. J Med Biochem. 2023 Mar 15;42(2):258–64. DOI: 10.5937/jomb0-39570. PMID: 36987412. PMCID: PMC10040197.
  16. Zhang Y, Sun Y, He Z, Xu S, Liu C, Li Y, et al. Age-specific thyrotropin references decrease over-diagnosis of hypothyroidism in elderly patients in iodine-excessive areas. Clin Endocrinol (Oxf). 2021 Sep 28; DOI: 10.1111/cen.14589. PMID: 34585413.
  17. Cellini M, Santaguida MG, Virili C, Capriello S, Brusca N, Gargano L, et al. Hashimoto’s thyroiditis and autoimmune gastritis. Front Endocrinol (Lausanne). 2017 Apr 26;8:92. DOI: 10.3389/fendo.2017.00092. PMID: 28491051. PMCID: PMC5405068.
  18. Obermeyer Z, Samra JK, Mullainathan S. Individual differences in normal body temperature: longitudinal big data analysis of patient records. BMJ. 2017 Dec 13;359:j5468. DOI: 10.1136/bmj.j5468. PMID: 29237616. PMCID: PMC5727437.

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