The Thyroid Tune Up handout explains five common thyroid blood tests in simple terms and how they work together. The thyroid is a small gland located in the neck that produces hormones, mainly T4 and T3, which play a key role in regulating energy use, body temperature, heart rate, and many other essential body functions.
Thyroid hormones regulate energy use, weight, heart rate, and body temperature; imbalances can impact your daily well-being.
Symptoms like fatigue, mood changes, or unexplained weight shifts may be linked to an underactive or overactive thyroid.
Antibody testing helps uncover if your immune system is affecting thyroid health, even before significant symptoms appear.
Women aged 30 and above.
Those with thyroid concerns, inflammation, or vitamin deficiencies.
Measures the hormone that regulates thyroid function. Important for women because thyroid imbalances can affect weight, energy, mood, menstrual cycles, and fertility.
The Free Thyroxine (Free T4) test measures the primary hormone released by the thyroid, with “free” referring to the active form that is not bound to proteins in the blood. It is essential because Free T4 indicates the amount of thyroid hormone available for the body to utilize and is preferred over total T4 when evaluating thyroid function. Results are interpreted in conjunction with TSH: a low Free T4 with high TSH confirms hypothyroidism (underactive thyroid), while a high Free T4 with low TSH supports hyperthyroidism (overactive thyroid). Free T4 is also helpful in establishing whether an abnormal TSH result truly reflects a thyroid disorder.
The Free Triiodothyronine (Free T3) test measures the active thyroid hormone that has substantial effects on body cells, most of which is produced from T4 in other tissues. It is imperative because, in some cases of hyperthyroidism, T3 levels rise before T4 levels; therefore, measuring T3 can help evaluate thyroid function when TSH levels are low. Results are used in context: a high Free T3 with a low TSH suggests hyperthyroidism, even if Free T4 remains normal (sometimes referred to as T3-predominant hyperthyroidism). Conversely, in hypothyroidism, T3 may still appear normal, making it less useful when TSH is high. It is also important to note that Free T3 results can be less consistent than total T3 in some laboratories; therefore, clinics may choose one or the other depending on their reliability. Additionally, T3 is often low during serious non-thyroid illness, which does not necessarily indicate a thyroid disorder.
The Reverse Triiodothyronine (Reverse T3 or rT3) test measures an inactive form of T3 that the body produces from T4, which does not activate thyroid hormone receptors in the same way as active T3. rT3 levels can rise during serious illness (known as “non-thyroidal illness syndrome”) or with certain medications, such as amiodarone. However, routine testing is generally not necessary in standard thyroid evaluations and is not used to diagnose typical hypothyroidism or hyperthyroidism. In special situations, research and specialty clinics may measure rT3, for example, in suspected rare enzyme or transport disorders, or to study hormone patterns during illness. Recent findings also indicate that rT3 tends to track with free T4/T3 and TSH in individuals receiving thyroid hormone therapy, although this does not alter standard care for most patients.
The Thyroid Peroxidase (TPO) Antibodies test detects antibodies immune proteins that target thyroid peroxidase, an enzyme essential for the thyroid's hormone production. A positive result suggests that the immune system is involved. This test is necessary because TPO antibodies support the diagnosis of autoimmune thyroid disease: they are commonly found in Hashimoto’s disease, a frequent cause of an underactive thyroid, and can also be present in Graves’ disease, which causes an overactive thyroid.
