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How is hypothyroidism detected through blood tests?

May 13, 2025 Source: Cainiu Health
Dr. Yang Chuanjia
Introduction
In general, hypothyroidism can be detected through blood tests, including measuring levels of thyroid-stimulating hormone (TSH), free thyroxine (FT4), total thyroxine (T4), total triiodothyronine (T3), and testing for thyroid autoantibodies.

Generally, hypothyroidism is a disease characterized by reduced body metabolism caused by decreased synthesis or secretion of thyroid hormones or insufficient physiological effects thereof. Hypothyroidism can be detected through blood tests, which include measuring levels of thyroid-stimulating hormone (TSH), measuring free thyroxine (FT4) levels, assessing total thyroxine (T4) levels, measuring total triiodothyronine (T3) levels, and testing for thyroid autoantibodies. Detailed analysis is as follows:

1. Measuring Thyroid-Stimulating Hormone (TSH) Levels

TSH is the most sensitive indicator for diagnosing hypothyroidism. The normal range is generally 0.4–4.0 mIU/L. In patients with hypothyroidism, TSH levels are typically significantly elevated due to insufficient thyroid hormones in the body.

2. Measuring Free Thyroxine (FT4) Levels

Free thyroxine (FT4) reflects the level of active thyroid hormone not bound to proteins in the blood. The normal range is approximately 0.8–1.8 ng/dL. In patients with hypothyroidism, FT4 levels are usually decreased due to insufficient thyroid function.

3. Measuring Total Thyroxine (T4) Levels

Total thyroxine (T4) indicates the total level of thyroxine in the serum, including both protein-bound and unbound fractions. In patients with hypothyroidism, T4 levels are typically also low and are significantly influenced by serum protein levels.

4. Measuring Total Triiodothyronine (T3) Levels

Total triiodothyronine (T3) is also an indicator reflecting thyroid function, similar to total thyroxine (T4). In patients with hypothyroidism, total triiodothyronine (T3) levels are usually decreased.

5. Measuring Thyroid Autoantibodies

Measuring thyroid autoantibodies includes testing for anti-thyroid peroxidase antibodies (TPOAb) and thyroglobulin antibodies (TgAb), commonly seen in autoimmune hypothyroidism. Elevated levels suggest an autoimmune attack, leading to reduced thyroid function.

Blood tests measuring TSH, FT4, T4, T3, and thyroid autoantibodies can accurately assess thyroid function and diagnose hypothyroidism. Once diagnosed with hypothyroidism, patients should actively follow the treatment plan recommended by their physician, such as thyroid hormone replacement therapy, to maintain normal thyroid function and improve quality of life.

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