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- Sample Types Serum, Plasma, Urine, Fecal Extracts, Tissue Culture Media
- Species Thyroxine is identical across species
- Range 50-0.8 ng/mL or 4,000-63 pg/mL
- Time to Answer 90 Minutes
- Samples/Kit 40 or 232 in Duplicate
- Readout Colorimetric, 450 nm
- Standard Curve
The thyroid hormones, triiodothyronine (T3) and thyroxine (T4) are tyrosine-based hormones produced by the thyroid gland that are primarily responsible for regulation of metabolism. Iodine is necessary for the production of T3 and T4. A deficiency of iodine leads to decreased production of T3 and T4, enlarges the thyroid tissue and will cause the disease known as goitre. The major form of thyroid hormone in blood is T4, which has a longer half-life than T3. The ratio of T4 to T3 released into the blood is roughly 20 to 1. T4 is converted to the active T3 (three to four times more potent than T4) within cells by deiodinases. All three deiodinase isoforms are selenium-containing enzymes, thus dietary selenium is essential for T3 production.
Hypothyroidism is the condition that results from under-production of thyroxine by the thyroid gland either because the gland is naturally underactive or because radioiodine therapy or surgery for an overactive gland has resulted in underactivity. Thyroxine is commonly taken to replace the deficiency which exists in such situations and therefore to restore normal metabolic activity. The concentration of T3 and T4 in the blood regulates the pituitary release of thyrotropin in a negative feedback loop such that when T3 and T4 concentrations are high, TSH production is reduced.