Hypothyroidism is a condition where the thyroid gland, located in the neck, does not produce enough thyroid hormones. These hormones are important for regulating the body’s metabolism and energy use. Accurate diagnosis through blood tests is a process for understanding thyroid function and guiding appropriate management. These tests measure specific hormone levels to determine if the thyroid is functioning optimally.
The Initial Screening Test
The initial and most common blood test for assessing thyroid function measures Thyroid-Stimulating Hormone (TSH). TSH is produced by the pituitary gland and acts as a signal to the thyroid, telling it how much thyroid hormone to produce. When thyroid hormone levels in the blood are low, the pituitary gland releases more TSH to stimulate the thyroid to work harder. Therefore, a high TSH level often indicates an underactive thyroid, as the pituitary is attempting to increase hormone production. Conversely, a low TSH level can suggest an overactive thyroid. This test often shows changes before actual thyroid hormone levels become significantly abnormal, serving as an early indicator of potential thyroid dysfunction.
Further Diagnostic Tests
If TSH levels are abnormal, additional blood tests are often ordered to provide a more complete picture of thyroid function. Free Thyroxine (Free T4) is one such test, measuring the active, unbound form of the main thyroid hormone circulating in the blood. Its “free” portion is the hormone available to enter and affect body tissues. Another test is Free Triiodothyronine (Free T3), which measures the active form of T3, largely converted from T4 in the body. While Free T4 is a more direct reflection of thyroid gland function in most cases, Free T3 can sometimes be measured, though it is less commonly used for diagnosing hypothyroidism.
Thyroid antibody tests are also important, particularly when an autoimmune cause for thyroid dysfunction is suspected. In autoimmune conditions, these antibodies mistakenly target the thyroid gland. Two common types measured are thyroid peroxidase antibodies (anti-TPO) and thyroglobulin antibodies (anti-Tg). Detecting elevated levels of these antibodies can help identify conditions like Hashimoto’s thyroiditis, which is a common cause of hypothyroidism.
What Test Results Mean
A high TSH level, typically above 4.0 milliunits per liter (mU/L), commonly indicates an underactive thyroid, also known as primary hypothyroidism. This elevation signifies that the pituitary gland is working overtime to stimulate a sluggish thyroid. If a high TSH is accompanied by a low Free T4 level, it confirms a diagnosis of primary hypothyroidism.
While Free T3 levels can also be low in hypothyroidism, the Free T4 measurement is generally considered more reliable for diagnosis because T3 levels are often the last to become abnormal. In some cases, TSH might be elevated while Free T4 and Free T3 remain within normal ranges, indicating a condition known as subclinical hypothyroidism.
The presence of thyroid antibodies, such as elevated anti-TPO or anti-Tg, suggests an autoimmune origin for the hypothyroidism, such as Hashimoto’s thyroiditis. A positive antibody test, especially anti-TPO, is found in a significant majority of individuals with Hashimoto’s disease. However, some individuals may test positive for these antibodies without having overt thyroid disease, indicating an increased future risk. A healthcare professional reviews all test results in the context of an individual’s overall health and symptoms to make an accurate diagnosis.
After Your Blood Test
If hypothyroidism is confirmed, the typical approach to management involves thyroid hormone replacement therapy. This usually means taking a daily medication, such as levothyroxine, which is a synthetic form of the T4 hormone. This medication helps restore hormone levels to a normal range, aiming to alleviate symptoms.
Treatment with levothyroxine is often a long-term commitment. Regular monitoring through follow-up blood tests, typically measuring TSH levels, is important to ensure the medication dosage remains appropriate. These tests help the healthcare provider adjust the dosage to maintain stable hormone levels and optimize well-being. Adjustments in medication may be needed periodically to ensure continued effectiveness.