Thyroid Hormone Testing Guide: A Comprehensive Review of How to Test for Thyroid Hormones

The thyroid is a small, butterfly-shaped gland located in the lower front of the neck, just above the collarbone. Despite its small size, the thyroid plays a crucial role in regulating various bodily functions, primarily by producing and releasing two main hormones: thyroxine (T4) and triiodothyronine (T3). Thyroid hormones impact nearly every bodily function, including metabolism, cardiovascular function, body temperature regulation, growth and development, reproductive health, and digestion. Disorders of the thyroid gland can lead to a range of health issues due to the disruption of hormone regulation and production. Therefore, proper diagnosis and management of thyroid diseases are essential to ensure optimal overall health and well-being. This is a health area where functional medicine thrives. (11) 

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Understanding Thyroid Hormones

The hypothalamus and pituitary gland regulate the production and release of thyroid hormones through a feedback loop called the hypothalamus-pituitary-thyroid (HPT) axis. The HPT axis regulates the body's metabolism, energy production, temperature regulation, and other physiological processes. Disorders within this axis can lead to thyroid-related conditions, which can have wide-ranging effects on health. Here's how the HPT axis works:

The process begins in the hypothalamus, a region of the brain that senses the body's need for thyroid hormones. When thyroid hormone levels in the blood drop below a certain threshold, the hypothalamus releases a hormone called thyrotropin-releasing hormone (TRH). (3, 11) 

TRH travels from the hypothalamus to the pituitary gland, a small gland located at the base of the brain. In response to TRH, the pituitary gland releases thyroid-stimulating hormone (TSH) into the bloodstream. (3, 11) 

TSH then travels to the thyroid gland, which is located in the neck. TSH binds to receptors on the thyroid gland's cells, stimulating the production and release of thyroid hormones: T4 and T3. The thyroid gland primarily produces T4, a less active thyroid hormone. T4 is released into the bloodstream and circulates throughout the body. (3, 11) 

Most of the T4 is converted into the more active form of thyroid hormone, T3, in various tissues, including the liver, kidneys, and other cells. T3 is the thyroid hormone that exerts most biological effects on the body's cells and tissues. (3, 11) 

As T3 and T4 levels increase in the blood, they exert negative feedback on both the hypothalamus and the pituitary gland. This feedback inhibits the release of TRH and TSH, helping to maintain thyroid hormone levels within a narrow and optimal range. (3, 11)

Common Thyroid Disorders and Testing

Thyroid disorders are conditions that affect the health and function of the thyroid gland. Thyroid disorders often present with vague symptoms that can overlap with other conditions. Accurate thyroid hormone testing is critical in diagnosing and managing thyroid disorders, ensuring timely and appropriate interventions to improve the patient's quality of life and prevent potential health complications.

Thyroiditis

Thyroiditis refers to inflammation of the thyroid gland, which can lead to various disruptions in thyroid hormone production and release. There are eight thyroiditis types: Hashimoto's, silent, postpartum, radiation-induced, subacute (De Quervain's), acute infectious, drug-induced, and Reidel's. 

Hypothyroidism

Hypothyroidism occurs when the thyroid gland doesn't produce enough thyroid hormones. Hashimoto's is an autoimmune disease and the most common cause of hypothyroidism. It occurs when immune proteins, called autoantibodies, target self-proteins and impair the thyroid's ability to produce hormones. Subclinical hypothyroidism is a precursor to overt hypothyroidism, in which lab work reveals an elevated TSH with T4 and T3 levels in the normal range.

Hyperthyroidism

In hyperthyroidism, the thyroid gland produces excessive amounts of thyroid hormones. This can lead to symptoms such as weight loss, rapid heartbeat, anxiety, heat intolerance, and trembling. If left untreated, hyperthyroidism can result in serious health problems, including heart issues and bone loss. Graves' disease causes four out of five hyperthyroid cases in the United States. In Graves' disease, the immune system makes thyroid-stimulating immunoglobulin (TSI), which acts like TSH and causes the thyroid to overproduce thyroid hormone.

Indications for Thyroid Hormone Testing

Thyroid hormone testing is essential for diagnosing thyroid disorders and ensuring proper management of thyroid health. Various symptoms and risk factors may indicate the need for thyroid hormone testing.

Symptoms that may warrant thyroid hormone testing include:

• Unexplained fatigue, weakness, and a general feeling of sluggishness
• Significant and unexplained weight gain or loss
• Intolerance to cold or excessive sweating and heat sensitivity 
• An irregular or rapid heart rate or palpitations
• Irregular menstrual cycles, heavy periods, or other menstrual changes
• Brittle hair, hair loss, dry skin, and changes in skin texture
• Constipation and diarrhea
• Mood disturbances, depression, anxiety, and difficulty concentrating
• Aches, pains, and stiffness in muscles and joints
• A visibly enlarged thyroid gland (goiter), discomfort/swelling in the front of the neck, and difficulty swallowing

Risk factors that may indicate the need for thyroid hormone testing include: 

• A family history of thyroid disorders increases the risk of developing similar issues
• Age over 60
• Women are more likely than men to develop thyroid disorders, especially autoimmune conditions like Hashimoto's and Graves'
• Having a personal history of concomitant autoimmune conditions, such as type 1 diabetes or celiac disease
• Pregnancy and the postpartum period
• Previous exposure to radiation in the neck area or a history of thyroid surgery 
• Certain medications, like lithium and amiodarone, can affect thyroid function
• Both too little and excessive iodine intake can negatively impact thyroid function

Types of Thyroid Tests Used in Functional Medicine

An array of thyroid tests are employed by functional medicine doctors to comprehensively evaluate thyroid health, the HPT axis, and its impact on overall well-being. Conventionally, most doctors only measure TSH as a marker of thyroid health; however, it has been estimated that measuring TSH alone without any other markers of thyroid function can miss about 7% of thyroid dysfunction. Therefore, functional and integrative medicine doctors usually order a comprehensive thyroid panel, which includes multiple hormones and markers related to the thyroid function cascade. This allows a better understanding of thyroid function and earlier detection of thyroid dysfunction. (3) 

Thyroid Stimulating Hormone (TSH)

TSH plays a central role in regulating thyroid function. It is a primary marker for screening thyroid function due to its sensitive responsiveness to changes in thyroid hormone levels. Elevated TSH levels suggest a hypothyroid state. On the other hand, low TSH levels indicate hyperthyroidism. (27)

Triiodothyronine (T3)

Free and total T3 are different measurements of triiodothyronine. Monitoring free and total T3 levels provides additional information about thyroid function beyond what can be inferred from TSH alone. 

‍Total T3 measures the total amount of T3 in the bloodstream, including both the T3 bound to proteins and the small amount that is unbound. Total T3 represents the thyroid's ability to produce T3 but doesn't directly reflect the amount of biologically active T3 available to the body. (3) 

Free T3 measures the unbound, biologically active form of T3 available for immediate use by the body's cells. Free T3 is considered the more accurate indicator of thyroid function since it represents the portion of T3 that can exert its metabolic effects. (3) 

Monitoring both free and total T3 levels can provide a more comprehensive understanding of thyroid function. In hypothyroidism, free and/or T3 levels may be low, indicating a decrease in the active form of thyroid hormone available to the body's tissues. In hyperthyroidism, free T3 levels may be elevated due to excess production of thyroid hormones. (26) 

Thyroxine (T4)

About 90% of hormone produced by the thyroid is thyroxine (3). Just as with T3, measuring the total and free portions of T4 in circulation can be important to assess thyroid health. It is common to see low levels of free and total T4 in hypothyroidism and high levels of free and total T4 in hyperthyroidism (25). 

Total T4 measures the total amount of T4 in the bloodstream, including both the T4 that is bound to proteins (such as thyroxine-binding globulin) and the smaller percentage that is unbound (3, 25).

Free T4 measures the unbound, biologically active form of T4 readily available for conversion to the more active T3 or interaction with target cells. Free T4 is a more accurate indicator of thyroid function, as it represents the portion of T4 that can exert its effects. (3, 25)

Reverse T3 (rT3) 

rT3 is a predominantly biologically inactive form of thyroid hormone produced through the conversion of T4. Conversion of T4 to rT3 is preferred over the T4-T3 conversion pathway in response to various physiological stressors, such as illness, chronic stress, and nutritional deficiencies. Too much rT3 can be problematic because it competes with T3 at receptor sites, reducing the availability of active T3 for cellular functions and causing hypothyroid-like symptoms. (3, 14) 

Thyroid Antibodies

Thyroid antibodies are proteins produced by the immune system that target various components of the thyroid gland. These antibodies in the blood indicate an autoimmune response, where the immune system mistakenly attacks the thyroid tissue. 

Thyroid peroxidase antibodies (TPOAb) target an enzyme called thyroid peroxidase, which is involved in producing thyroid hormones. Elevated TPOAb levels are associated with Hashimoto's thyroiditis, present in over 80% of cases.

Thyroglobulin is a protein the thyroid gland produces during thyroid hormone synthesis. Elevated thyroglobulin antibodies (TgAb) are also seen in Hashimoto's thyroiditis in over 50% of cases.

Thyroid Stimulating Immunoglobulins (TSI) stimulate the TSH receptor on thyroid cells, leading to an overproduction of thyroid hormones. Elevated TSI levels are a hallmark of Graves' disease.

Research indicates that an increasing number of healthy Americans have autoimmune antibodies in circulation, meaning autoimmune processes occur years before clinical symptoms and disease appear. Regarding thyroid autoimmunity, studies have reported that over 13% of adults were TPOAb positive, and 11.5% were TgAb positive.

Interpreting Thyroid Test Results Using Functional Medicine

Interpreting thyroid test results using functional medicine involves looking beyond conventional reference ranges and considering optimal lab values that align with a patient's overall health and well-being. The chart below documents conventional and functional medicine reference ranges for thyroid biomarkers. Optimal lab ranges consider the patient's lab results and symptoms rather than simply whether results fall within conventional norms. Abnormal levels may indicate imbalances, stressors, or potential underlying conditions that need further investigation and intervention.

Other Thyroid Tests Used in Functional Medicine

In addition to the comprehensive thyroid panel, functional medicine doctors often order additional labs and imaging for patients with thyroid autoimmunity, dysfunction, and disease. These additional tests aim to identify the underlying factors responsible for disrupting the HPT axis, causing autoimmunity, and creating inflammation within the thyroid gland. 

A thyroid ultrasound is a non-invasive and painless imaging procedure that is usually ordered when enlargement and tenderness of the thyroid gland are noted on a physical exam. Ultrasound can help diagnose and monitor thyroid nodules, cysts, cancer, and inflammation contributing to thyroid disease. (24) 

Gluten sensitivity, intestinal dysbiosis, and micronutrient deficiencies are all factors known to contribute to thyroid autoimmunity, hypothyroidism, and hyperthyroidism. Therefore, gluten sensitivity panels, comprehensive stool analyses, and micronutrient panels are specialty tests routinely ordered in a functional evaluation of thyroid disease. 

Because thyroid disease can also increase the risk of cardiovascular disease and osteoporosis, your doctor will likely also recommend tests like hs-CRP, comprehensive lipid panel, and DEXA scan as part of a preventive health screening. 

Next Steps After Thyroid Hormone Testing

If your functional medicine doctor has detected thyroid dysfunction based on a comprehensive thyroid evaluation, they will recommend an integrative treatment plan to correct imbalances and restore thyroid function. While some people may require replacement thyroid hormones, such as levothyroxine and liothyronine, to bring thyroid hormones back into optimal range, a variety of natural products can also support thyroid health. 

Selenium

Selenium is an essential component of selenoproteins that participate in critical body functions, including DNA synthesis, reproduction, immune responses, and thyroid hormone metabolism (18). Selenium is found in many dietary sources, including beef, poultry, seafood, and Brazil nuts. Supplementing selenium in doses of 200 mcg daily has been shown to reduce TPO antibodies by up to 63.6%, normalize thyroid hormones, and improve hypothyroid and hyperthyroid symptoms. 

Vitamin D

Vitamin D is imperative in modulating a healthy immune response and inflammation and reducing the risk of autoimmune disease. Human studies have also found a correlation between lower vitamin D levels and hypothyroidism. 

Ashwagandha

Ashwagandha is an adaptogenic herb that helps improve resilience to stress, balances thyroid hormone conversion and levels, and has anti-inflammatory effects (21). Studies have demonstrated the positive impact of administering ashwagandha in hypothyroid patients. 

L-Carnitine

L-carnitine is an amino acid that blocks the utilization of thyroid hormones in the body's tissues, proving helpful in treating hyperthyroid symptoms, including weakness, fatigue, shortness of breath, heart palpitations, insomnia, tremors, and nervousness (7, 21).  

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Summary

If you are one of the individuals experiencing debilitating symptoms indicating thyroid dysfunction, despite your conventional doctor telling you your thyroid labs are "normal," you may benefit from a more comprehensive and functional evaluation of thyroid health. Functional medicine doctors evaluate the complete HPT axis with various thyroid function tests that go beyond the standard TSH screening. Functional medicine emphasizes the importance of optimal lab values that align with individual health goals and offers personalized care and proactive strategies to maintain optimal thyroid health.