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Interpret thyroid function tests including TSH, Free T4, and Free T3 to assess for hypothyroidism, hyperthyroidism, and subclinical thyroid disorders.
Normal range: 0.4-4.0 mIU/L
Normal range: 0.8-1.8 ng/dL
Normal range: 2.3-4.2 pg/mL
| Test | Normal Range | Units |
|---|---|---|
| TSH | 0.4 - 4.0 | mIU/L |
| Free T4 | 0.8 - 1.8 | ng/dL |
| Free T3 | 2.3 - 4.2 | pg/mL |
Note: Reference ranges may vary slightly between laboratories. Always use the reference range provided by your specific laboratory.
| TSH | Free T4 | Diagnosis |
|---|---|---|
| High | Low | Primary hypothyroidism |
| High | Normal | Subclinical hypothyroidism |
| Low | High | Primary hyperthyroidism |
| Low | Normal | Subclinical hyperthyroidism |
| Low | Low | Central hypothyroidism |
| Normal | Normal | Euthyroid (normal) |
Thyroid function testing is essential for diagnosing and managing thyroid disorders, which affect millions of people worldwide. The thyroid gland produces hormones that regulate metabolism, energy levels, body temperature, and many other vital functions.
TSH (Thyroid Stimulating Hormone): Produced by the pituitary gland, TSH stimulates the thyroid to produce T4 and T3. It's the most sensitive test for detecting thyroid dysfunction. When thyroid hormone levels are low, TSH increases; when hormone levels are high, TSH decreases.
Free T4 (Thyroxine): The main hormone produced by the thyroid gland. "Free" T4 refers to the hormone not bound to proteins, representing the biologically active form available to tissues.
Free T3 (Triiodothyronine): The more active thyroid hormone, mostly converted from T4 in peripheral tissues. It's particularly useful in diagnosing hyperthyroidism and monitoring certain thyroid conditions.
Hypothyroidism: Underactive thyroid causing fatigue, weight gain, cold intolerance, constipation, dry skin, and depression. Primary hypothyroidism is most common, often due to Hashimoto's thyroiditis.
Hyperthyroidism: Overactive thyroid causing weight loss, anxiety, heat intolerance, palpitations, tremor, and increased appetite. Common causes include Graves' disease and toxic nodular goiter.
Subclinical Thyroid Disease: Abnormal TSH with normal T4, often asymptomatic but may progress to overt disease. Treatment decisions depend on TSH level, symptoms, and cardiovascular risk.
Testing is recommended if you have symptoms of thyroid dysfunction (fatigue, weight changes, temperature intolerance), family history of thyroid disease, during pregnancy or when planning pregnancy, if taking medications affecting thyroid function (lithium, amiodarone), or as screening for those over 60, especially women. Annual monitoring is needed if you're on thyroid medication.
This pattern indicates subclinical hypothyroidism, where the thyroid is working harder to maintain normal hormone levels. The pituitary produces more TSH to compensate for early thyroid dysfunction. Treatment is typically considered if TSH is > 10 mIU/L, you have symptoms, positive thyroid antibodies, or cardiovascular risk factors. Many cases are monitored without immediate treatment.
Yes, several medications can affect thyroid tests. Biotin (vitamin B7) supplements can cause falsely abnormal results and should be stopped 2-3 days before testing. Lithium, amiodarone, and interferon can cause actual thyroid dysfunction. Estrogen and birth control pills increase thyroid-binding proteins, potentially affecting total T4 (but not free T4). Always inform your doctor of all medications and supplements.
Fasting is not required for thyroid function tests. However, if you're taking levothyroxine, you should take your morning dose after the blood draw, not before, as it can temporarily elevate T4 levels. Testing should be done at consistent times (preferably morning) for reliable comparison between tests, especially when monitoring treatment.
Low TSH with low T4 suggests central (secondary or tertiary) hypothyroidism, indicating a problem with the pituitary gland or hypothalamus rather than the thyroid itself. This is less common but more serious, requiring evaluation of pituitary function with MRI and testing of other pituitary hormones. It may be associated with other hormone deficiencies and requires endocrinology referral.
After starting or changing levothyroxine dosage, recheck TSH and Free T4 in 6-8 weeks. Once stable on the correct dose, annual monitoring is typically sufficient for most patients. More frequent testing may be needed during pregnancy, with significant weight changes, when starting medications that interact with thyroid hormone, or if symptoms suggest poor control. Your doctor may adjust timing based on individual factors.
Free T3 is not routinely needed for initial diagnosis. It's most useful when diagnosing hyperthyroidism (especially T3-toxicosis where T3 is elevated but T4 is normal), monitoring patients on T3-containing medications, or evaluating patients with persistent symptoms despite normal TSH and T4. For routine hypothyroidism diagnosis and monitoring, TSH and Free T4 are usually sufficient.
Thyroid antibodies (anti-TPO, anti-thyroglobulin, TSH receptor antibodies) indicate autoimmune thyroid disease. TPO antibodies help confirm Hashimoto's thyroiditis in hypothyroidism. TSH receptor antibodies (TRAb) help diagnose Graves' disease in hyperthyroidism. Testing is useful when initially diagnosing thyroid dysfunction, with subclinical disease to assess progression risk, or during pregnancy. Once diagnosed, repeated antibody testing is usually not necessary.
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