Tests Thyroïdiens Avancés

TSH (Thyroid Stimulating Hormone) is produced by the pituitary gland and acts as a sensitive barometer of thyroid function. When your thyroid is underactive, the pituitary releases more TSH to stimulate hormone production, resulting in elevated levels. Conversely, an overactive thyroid suppresses TSH to very low levels. Because TSH responds to even minor changes in thyroid hormone levels, it detects dysfunction earlier than T3 or T4 measurements alone, making it the gold-standard initial screening test for thyroid disorders.

T4 (thyroxine) is the primary hormone produced by the thyroid gland and circulates in large quantities as a reservoir. T3 (triiodothyronine) is the biologically active form that directly influences metabolism, energy, and cellular function. Most T3 is converted from T4 in peripheral tissues. Free T3 and Free T4 tests measure the unbound, active portions available to your cells rather than the total amount, providing a more accurate picture of thyroid function. Some patients have normal T4 but impaired conversion to T3, which only a complete panel reveals.

Fasting is not required for thyroid tests. However, the timing of your blood draw matters significantly. TSH levels follow a circadian rhythm, peaking in the early morning hours and declining by midday. For the most accurate and consistent TSH results, we recommend testing before 10 AM. If you take thyroid medication such as levothyroxine, take your dose after the blood draw rather than before, as taking it beforehand can temporarily elevate Free T4 and affect interpretation.

Thyroid antibodies are proteins produced by the immune system that mistakenly attack the thyroid gland. Thyroid Peroxidase Antibodies (TPO Ab) and Thyroglobulin Antibodies (TG Ab) are markers of autoimmune thyroid disease. Elevated TPO antibodies are found in approximately 90 percent of Hashimoto's thyroiditis cases and 70 percent of Graves' disease cases. Testing is recommended when TSH is abnormal, when there is a family history of autoimmune thyroid disease, or when thyroid function fluctuates. Positive antibodies with normal TSH indicate increased risk of future thyroid dysfunction.

Testing frequency depends on your risk profile and current status. Healthy adults with no symptoms or risk factors should consider screening every five years starting at age 35. Women planning pregnancy should test beforehand. Patients on thyroid medication typically need testing every six to eight weeks after dose changes and every six to twelve months once stable. Those with positive thyroid antibodies but normal function should test annually. Your endocrinologist will recommend a personalised schedule based on your specific situation.

Yes, high-dose biotin, commonly found in hair, skin, and nail supplements, can significantly interfere with immunoassay-based thyroid tests. Biotin can cause falsely low TSH readings and falsely elevated Free T3 and Free T4, potentially mimicking hyperthyroidism. Standard multivitamins contain low biotin doses that typically do not cause issues, but standalone biotin supplements at 5,000 to 10,000 micrograms can affect results. We recommend stopping high-dose biotin at least 48 to 72 hours before your thyroid blood test to ensure accuracy.

Subclinical hypothyroidism is a condition where TSH is mildly elevated but Free T4 remains in the normal range. Patients may have no symptoms or subtle ones like mild fatigue or weight gain. Treatment decisions depend on the degree of TSH elevation, presence of symptoms, thyroid antibody status, age, and cardiovascular risk factors. TSH above 10 mIU/L generally warrants treatment. For mild elevations between 5 and 10 mIU/L, particularly with positive antibodies, treatment may be recommended. Regular monitoring is essential for those not started on medication.

Pregnancy significantly impacts thyroid function due to increased hormone demands and the effects of hCG (human chorionic gonadotropin), which stimulates the thyroid. TSH reference ranges are lower during pregnancy, particularly in the first trimester. Untreated hypothyroidism during pregnancy increases the risk of miscarriage, preeclampsia, premature birth, and impaired fetal brain development. All pregnant women should have thyroid screening, ideally in the first trimester. Women on levothyroxine typically need a 25 to 50 percent dose increase during pregnancy, requiring frequent monitoring.