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In addition to an increase level of TSH above 2.5 mU/L during the first trimester, and above 3 mU/L in the second and third trimester, a decrease fT4 concentration (also using reference range for pregnant women) will help clinicians diagnose overt hypothyroidism during pregnancy.
1 Thyroxine (T4) and Triiodothyronine (T3) . 2  Thyroxine (T4) is the key hormone produced in the thyroid gland. 3  Low levels of T4 produce hypothyroidism, and high levels produce hyperthyroidism. 4  Thyroxine converts to triiodothyronine (T3), which is a more biologically active hormone.ypothyroidism is a condition marked by low thyroxine (T4) hormone levels, and a test can measure levels of this hormone in the blood. However, this test is usually inadequate for the following reasons: 1  T4 levels can be normal early in the disease process leading to hypothyroidism.