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Jostel's TSH index (TSHI or JTI), also referred to as Jostel's thyrotropin index or Thyroid Function index (TFI), is a method for estimating the thyrotropic (i.e. thyroid stimulating) function of the anterior pituitary lobe in a quantitative way. [1] [2] The equation has been derived from the logarithmic standard model of thyroid homeostasis.
It is the primary form of thyroid hormone found in the blood and acts as a prohormone of the more active thyroid hormone, triiodothyronine (T 3). [1] Thyroxine and its active metabolites are essential for regulating metabolic rate , supporting heart and muscle function , promoting brain development , and maintaining bone health .
Like during fetal development, thyroid hormone levels are low in the overloaded heart tissue in a local hypothyroid state, with low levels of deiodinase 1 and deiodinase 2. Although deiodinase 3 levels in a normal heart are generally low, in cardiomyopathy deiodinase 3 activity is increased to decrease energy turnover and oxygen consumption.
It is also beneficial for assessing the severity of already confirmed thyroid hormone resistance, [6] even on replacement therapy with L-T4, [7] and for monitoring the pituitary response to substitution therapy with thyromimetics (e.g. TRIAC) in RTH Beta. [8] In autoimmune thyroiditis the TTSI is moderately elevated. [9]
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 from T4, and breaks down T3.
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.
Calcitonin, a peptide hormone produced and secreted by the thyroid, is usually not included in the meaning of "thyroid hormone". Thyroid hormones are one of the factors responsible for the modulation of energy expenditure. This is achieved through several mechanisms, such as mitochondrial biogenesis and adaptive thermogenesis. [7]
3,3'-Diiodothyronine, also known as 3,3'-T 2, is a metabolite of thyroid hormone. It is formed from the breakdown of triiodothyronine . Levels can be affected in certain disease states.