Thyroid Function Tests: TSH, T3, and T4 Explained

Understanding Thyroid Function Tests

The thyroid is a small but vital gland that regulates metabolism, energy production, and body temperature. Thyroid function tests measure hormone levels to diagnose disorders like hypothyroidism and hyperthyroidism. These tests are essential for evaluating fatigue, weight changes, and other metabolic symptoms.

Key Thyroid Hormones

1. TSH (Thyroid Stimulating Hormone)

TSH is produced by the pituitary gland and controls thyroid hormone production. Elevated TSH suggests hypothyroidism (underactive thyroid), while low TSH indicates hyperthyroidism (overactive thyroid). TSH is typically the first test ordered to assess thyroid health.

2. Free T4 (Thyroxine)

T4 is the primary hormone produced by the thyroid gland. Free T4 measures the active, unbound hormone available to body tissues. Low T4 indicates hypothyroidism, while high T4 suggests hyperthyroidism.

3. Free T3 (Triiodothyronine)

T3 is formed from T4 and is the more active thyroid hormone. Free T3 tests help diagnose thyroid disorders and assess treatment effectiveness, particularly in cases of hyperthyroidism.

4. Thyroid Antibodies

These tests detect autoimmune thyroid diseases like Hashimoto's thyroiditis and Graves' disease, where the immune system attacks the thyroid gland.

Common Thyroid Disorders

• ✓ Hypothyroidism: Underactive thyroid causing fatigue, weight gain, cold sensitivity
• ✓ Hyperthyroidism: Overactive thyroid causing weight loss, anxiety, rapid heartbeat
• ✓ Hashimoto's Disease: Autoimmune condition causing hypothyroidism
• ✓ Graves' Disease: Autoimmune condition causing hyperthyroidism
• ✓ Thyroid Nodules: Abnormal growths requiring evaluation

Symptoms Warranting Thyroid Testing

Unexplained fatigue, unexplained weight changes, temperature sensitivity, hair loss, mood changes, dry skin, irregular heartbeat, and difficulty concentrating can all indicate thyroid dysfunction. If you experience these symptoms, thyroid function testing is recommended.

Normal Ranges

Normal values vary by laboratory:

• • TSH: 0.4-4.0 mIU/L
• • Free T4: 0.8-1.8 ng/dL
• • Free T3: 2.3-4.2 pg/mL

Preparation and Testing

No special preparation is required for thyroid function tests. A simple blood draw provides the sample needed. Results are typically available within 24-48 hours. Inform your healthcare provider about medications and supplements, as some can affect thyroid hormone levels.