A thyroid blood test can be a helpful diagnostic tool for healthcare providers and an early step toward providing effective treatment for patients suffering from thyroid conditions. Also known as thyroid function tests, thyroid blood tests measure a series of specific blood markers to evaluate how the thyroid gland is functioning and help diagnose thyroid conditions. (14)
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The thyroid gland
Located at the front of the neck, with an approximate weight of 10–20 g, the small but mighty thyroid gland gets its butterfly shape from a central isthmus and two vascular lobes located on either side of the trachea. (6) Through the production of two main hormones, tetraiodothyronine (thyroxine, T4) and triiodothyronine (T3), the follicular cells of the thyroid gland impact almost every tissue throughout the body and regulate fat, protein, and carbohydrate metabolism. (11) As the hormones produced by the thyroid gland are responsible for keeping the body warm, managing its energy, and ensuring that all major systems and organs are working well, patients may start to experience noticeable symptoms of thyroid disease, requiring further investigation, when the thyroid gland begins to malfunction. (11)
Thyroid conditions
Conditions of the thyroid gland result in the production of a hormone supply to the body that’s too much or too little. Examples of thyroid conditions include goiter, hyperthyroidism, hypothyroidism, thyroid nodules, and thyroiditis. (11)
Symptoms of thyroid conditions
When the thyroid gland is working well, the hormones it produces help to regulate breathing, body weight, digestion, heart rate, and mood. When hormone production becomes dysregulated, normal bodily functions can be disrupted, resulting in a variety of symptoms. (1) The following are examples of symptoms attributed to two common thyroid conditions.
Symptoms of hyperthyroidism (overactive thyroid):
- Changes in pace or rhythm of heartbeat
- Changes in menstrual patterns, including lighter, less frequent periods
- Decreased weight
- Difficulty falling asleep or staying asleep
- Excess sweating and intolerance to heat
- Increased appetite
- Increased number of bowel movements
- Issues with sleep, feelings of nervousness or irritability
- Muscle weakness and shaky hands
- Presence of a goiter (lump) in the neck (8)
Symptoms of hypothyroidism (underactive thyroid):
- Changes in frequency of bowel movements, including constipation
- Changes in menstruation, irregular or heavier periods
- Decreased heart rate
- Difficulties with fertility
- Dryness of hair and skin
- Episodes of depression
- Increased weight
- Intolerance of cold
- Muscle and joint pain
- Tiredness
- Thinning hair (13)
Thyroid testing
Upon the presentation of symptoms, clinicians should consider additional assessment to differentiate if these are related to thyroid disease or other causes. To help with differential diagnosis, thyroid screening may be ordered to help rule out or identify a series of conditions.
According to clinical recommendations, thyroid screening of asymptomatic adults, or those without evident risk factors for developing thyroid disease, isn’t currently recommended. This guidance is based on a lack of evidence pointing to the benefits of testing this group of individuals. (2) When addressing issues pertaining to thyroid health, healthcare providers will likely start with a physical examination of the patient, followed by one or more thyroid blood tests, with further imaging tests ordered if indicated. (14)
Thyroid blood tests
Thyroid blood tests, including the following, are usually ordered sequentially with results dictating if further blood tests are required. (14)
TSH test
Measuring thyroid-stimulating hormone (TSH) in the blood is usually the starting point in thyroid investigations. TSH is created in the pituitary gland and is inversely related to the amount of T4 and T3 produced by the thyroid gland. (10)
T4 tests
A blood test reporting a high level of T4 can indicate hyperthyroidism or an excess intake of thyroxine replacement medication. Conversely, if the level of T4 in the blood returns as low, this could indicate hypothyroidism.
T4 can be assessed by measuring total T4 and free T4. Total T4 measures the overall amount of T4 in the blood, including both the bound and free forms, while free T4 specifically measures the unbound T4 that’s available to enter and affect tissues.
T4 uptake is another diagnostic test used to evaluate thyroid function. It measures how much T4 is taken up by thyroid-binding proteins in the blood. It helps determine the amount of thyroid hormone-binding capacity available and is often used in conjunction with other thyroid tests.
It’s important to remember that certain factors, such as severe illness, pregnancy, and the use of various medications including oral contraceptives, can impact the levels of T4 present. (14)
T3 tests
T3 is a thyroid hormone derived from the conversion of T4 in a de-iodination step and is considered the most biologically active form in regulating metabolism. There are three tests used to measure T3: total T3, free T3, and reverse T3 (rT3). Total T3 measures all forms of T3 in the blood, both bound to proteins and unbound. Free T3 measures only the most biologically active, unbound hormone available to tissues. rT3 is an inactive form of the thyroid hormone often elevated in times of stress or illness, and it’s also derived from T4. If a patient’s T4 level returns within the normal range, but hyperthyroidism is still suspected, a T3 test can be used to confirm the diagnosis. (10)(12)
Thyroid antibody tests
A thyroid antibody test might be ordered if thyroid disease is indicated by the results of previous blood tests and symptoms suggest an autoimmune condition. By checking the levels of thyroid antibodies, autoimmune thyroid disorders like Graves’s disease and Hashimoto’s disease become easier to identify. (14) Common tests include the anti-thyroid peroxidase antibody (anti-TPO) test, which is elevated in Hashimoto’s, the thyroglobulin antibodies (TgAb) test, also associated with Hashimoto’s, and the thyroid-stimulating immunoglobulin (TSI) test, which can indicate Graves’s disease.
Additional thyroid blood tests to consider
The following foundational nutrition assessments can also aid in the evaluation of thyroid health.
Selenium
A crucial micronutrient for thyroid function, selenium is a key component in both the synthesis of thyroid hormones and the conversion of T4 into the more biologically active T3 hormone. (5)
Iron
In order for thyroid hormone synthesis and metabolism to occur, the presence of iron is required. If an iron deficiency is suspected, a complete blood count (CBC) done in combination with an iron panel (ferritin, transferrin saturation, TIBC, and serum iron) may be recommended. (4) The addition of the iron panel can help differentiate other underlying causes of CBC abnormalities such as folate or B12 insufficiencies.
Serum folate and B12
An evaluation of these basic markers for circulating folate and B12, using the results of a CBC and iron panel, can offer a clearer picture of a patient’s nutritional status. Additional testing of methylmalonic acid (MMA) and homocysteine levels can provide further information regarding the functional nutrition status of these nutrients. (15)
Zinc
A deficiency in zinc can affect the thyroid’s ability to function efficiently. Zinc also plays a role in the regulation of the thyroid and hormone synthesis. (3)
Vitamin D
As vitamin D deficiency has been linked to metabolic health, impaired thyroid function, and autoimmune thyroid disorders, a test to ensure adequate vitamin D levels can be important in the evaluation of thyroid health. (7)
Thyroid imaging tests
Imaging tests are typically reserved for cases in which there are abnormalities in blood tests or physical exams, or when there are symptoms suggestive of thyroid dysfunction or nodules. Imaging tests used in thyroid screening include:
- Computed tomography (CT scan)
- Ultrasonography (ultrasound)
- Nuclear scans (9)
The bottom line
The thyroid gland plays a crucial role in overall health and body function. It influences metabolism and energy production, gastrointestinal and cardiovascular function, the nervous and reproductive systems, and more. (11) In turn, thyroid health can be influenced by imbalances in any of these systems, as well as many environmental factors, such as stress and toxin exposure.
The rising prevalence of thyroid conditions should prompt healthcare providers to thoroughly screen patients with the appropriate clinical presentation to differentiate thyroid conditions from other underlying causes. (16)
A comprehensive assessment of thyroid health includes an evaluation of patient symptoms; a physical exam; foundational thyroid blood tests such as TSH, free T4, and T3; and advanced testing such as thyroid antibodies, thyroid hormone uptake, and nutritional status testing since several nutrients are essential for optimal thyroid function. Thyroid imaging studies can also aid in the assessment of thyroid function and the identification of thyroid conditions.
Clinicians should interpret thyroid blood test results in the context of the patient’s history, medications, concurrent illnesses, and lifestyle (particularly stress and adrenal dysfunction, which significantly impact thyroid function). A comprehensive assessment, based on a whole-person approach, may positively influence patient outcomes and quality of life and can be an important tool for healthcare providers.
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