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Thyroid function tests (TFTs) is a collective term for blood tests used to check the function of the thyroid. [1] TFTs may be requested if a patient is thought to suffer from hyperthyroidism (overactive thyroid) or hypothyroidism (underactive thyroid), or to monitor the effectiveness of either thyroid-suppression or hormone replacement therapy.
Free T3 and T4 measurements are important because certain drugs and illnesses can affect the concentrations of transport proteins, resulting in differing total and free thyroid hormone levels. There are differing guidelines for T3 and T4 measurements. Free T4 levels should be measured in the evaluation of hypothyroidism, and low free T4 ...
D1, D2, and D3 regulate the levels of T4, T3, and rT3. Three primary deiodinases are responsible for thyroid hormone conversion and breakdown. Type 1 (D1) deiodinates T4 to the biologically active T3, as well as the hormonally inactive and possibly inhibitory rT3. [3] [5] Type 2 (D2) converts T4 into T3, and breaks down rT3. D3 produces rT3 ...
The major form of thyroid hormone in the blood is thyroxine (T 4), whose half-life of around one week [4] is longer than that of T 3. [5] In humans, the ratio of T 4 to T 3 released into the blood is approximately 14:1. [6] T 4 is converted to the active T 3 (three to four times more potent than T 4) within cells by deiodinases (5′-deiodinase).
Thyroid function testing frequently reveals decreased thyroid stimulating hormone (TSH) and increased serum levels of triiodothyronine (T3) and thyroxine (T4) during the acute phase of the disease. [2] The intrathyroidal T3 and T4 levels are often reflected by the T3 to T4 ratio, which is typically less than 20 (ng/dL divided by μg/dL). [22]
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.
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In the United States hyperthyroidism affects about 1.2% of the population. [3] Worldwide, hyperthyroidism affects 2.5% of adults. [8] It occurs between two and ten times more often in women. [1] Onset is commonly between 20 and 50 years of age. [2] Overall the disease is more common in those over the age of 60 years. [1]