Thyroid Blood Tests

The thyroid is a small, butterfly-shaped gland in the neck that has an outsized influence on how the body functions. It produces hormones that regulate metabolism, heart rate, body temperature, mood, and energy levels. When the thyroid is not functioning optimally, whether due to being overactive or underactive, the effects can be wide-ranging and are often mistaken for other conditions. Thyroid blood tests are a reliable way to assess how well the gland is working and to identify whether thyroid function may be contributing to symptoms you are experiencing.

What Do Thyroid Blood Tests Measure?

Thyroid function is assessed through a combination of hormone measurements. The most commonly ordered tests include:

TSH (Thyroid-Stimulating Hormone)

A TSH blood test measures a hormone called thyroid-stimulating hormone. TSH is produced by the pituitary gland and signals the thyroid to produce its hormones. It is typically the first and most sensitive marker used to screen for thyroid dysfunction. A high TSH level suggests the thyroid is underactive (hypothyroidism), while a low TSH may indicate an overactive thyroid (hyperthyroidism).

Free T4 (Thyroxine)

T4 is the primary hormone produced by the thyroid gland. The "free" measurement refers to the portion that is unbound and available for use by the body's tissues. T4 is converted into the more active T3 in various organs. Low free T4 alongside a high TSH confirms hypothyroidism.

Free T3 (Triiodothyronine)

T3 is the active form of thyroid hormone. Some individuals have normal TSH and T4 levels but low T3, which can still produce symptoms. T3 testing is not always included in a standard panel, but may be ordered when symptoms persist despite normal TSH results.

Thyroid Antibodies (TPO and TgAb)

Thyroid peroxidase antibodies (TPO) and thyroglobulin antibodies (TgAb) are markers of autoimmune thyroid conditions such as Hashimoto's thyroiditis. These tests are particularly useful when TSH levels are borderline or when there is a family history of autoimmune thyroid disease.

Symptoms That May Prompt a Thyroid Function Test

Thyroid dysfunction can present in many ways, and symptoms often develop gradually. Common reasons a healthcare provider may recommend thyroid blood tests include:

Persistent fatigue or low energy that does not improve with rest, unexplained weight gain or difficulty losing weight, hair thinning or hair loss, sensitivity to cold temperatures, low mood or depressive symptoms, brain fog or difficulty concentrating, dry skin, constipation, and irregular menstrual cycles in women.

Conversely, an overactive thyroid may cause weight loss, rapid heartbeat, anxiety, heat intolerance, and sleep disturbances.

Understanding Your Thyroid Test Results

Reference ranges vary between laboratories. Always discuss your results with a qualified healthcare provider.