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Total T4 is measured to see the bound and unbound levels of T4. The total T4 is less useful in cases where there could be protein abnormalities. The total T4 is less accurate due to the large amount of T4 that is bound. The total T3 is measured in clinical practice since the T3 has decreased amount that is bound as compared to T4. [citation needed]
Also in normal pregnancies, adequate levels of maternal thyroid hormone are vital in order to ensure thyroid hormone availability for the foetus and its developing brain. [88] Congenital hypothyroidism occurs in every 1 in 1600–3400 newborns with most being born asymptomatic and developing related symptoms weeks after birth. [89]
At the cellular level, T 3 is the body's more active and potent thyroid hormone. [2] T 3 helps deliver oxygen and energy to all of the body's cells, its effects on target tissues being roughly four times more potent than those of T 4. [2] Of the thyroid hormone that is produced, just about 20% is T 3, whereas 80% is produced as T 4.
Hypothyroidism is diagnosed by looking at the free thyroxine (T4) levels in people with elevated TSH levels, and comparing the ratio between them. People with high TSH and low T4 get a diagnosis ...
Levothyroxine should be used to keep TSH levels within the normal range for that trimester. The first-trimester normal range is below 2.5 mIU/L and the second and third trimesters normal range is below 3.0 mIU/L. [16] [44] Treatment should be guided by total (rather than free) thyroxine or by the free T 4 index.
The therapeutic target range TSH level for patients on treatment ranges between 0.3 and 3.0 μIU/mL. [18] For hypothyroid patients on thyroxine, measurement of TSH alone is generally considered sufficient. An increase in TSH above the normal range indicates under-replacement or poor compliance with therapy.
People with hyperthyroid will typically "take up" higher than normal levels of radioiodine. Normal ranges for RAI uptake are from 10 to 30%. In addition to testing the TSH levels, many doctors test for T 3, Free T 3, T 4, and/or Free T 4 for more detailed results. Free T 4 is unbound to any protein in the blood.
When circulating in the body, T3 and T4 are bound to transport proteins. Only a small fraction of the circulating thyroid hormones are unbound or free, and thus biologically active. T3 and T4 levels can thus be measured as free T3 and T4, or total T3 and T4, which takes into consideration the free hormones in addition to the protein-bound hormones.