Medicine

Reverse triiodothyronine (reverse T3, or rT3) is a molecule which is an isomer of triiodothyronine (T3). It is derived from thyroxine (T4) through the use of deiodinase.rT3, unlike T3, does not stimulate thyroid hormone receptors. However, rT3 nonetheless binds to these receptors, thereby blocking the action of T3. Under stress conditions, the adrenal glands produce excess amounts of cortisol. Cortisol inhibits the conversion of T4 to T3, thus shunting T4 conversion from T3 towards rT3. Consequently, there is a widespread shutdown in T3 binding across the body. This condition is termed Reverse T3 Dominance. It results in reduced body temperature,

The T3 (and T4) bind to nuclear receptors, thyroid receptors. However, T3 (and T4) are not very lipophilic and as a result, are unable to pass through the phospholipid bilayers. They therefore have specific transport proteins on the cell membranes of the effector organs which allow the T3 and T4 to pass into the cells. The thyroid receptors bind to response elements in gene promoters and thus enabling them to activate or inhibit transcription. The sensitivity of a tissue to T3 is modulated through the thyroid receptors.

Thyroid hormones (T4 and T3) are produced by the follicular cells of the thyroid gland and are regulated by TSH made by the thyrotrophs of the anterior pituitary gland. Because the effects of T4 in vivo are mediated via T3 (T4 is converted to T3 in target tissues; T3 is 3- to 5- fold more active than T4). Thyroxine (3,5,3',5'-tetraiodothyronine) is produced by follicular cells of the thyroid gland. It is produced as the precursor thyroglobulin (this is not the same as TBG), which is cleaved by enzymes to produce active T4. Thyroxine is produced by attaching iodine atoms to the ring

Liothyronine sodium is the L-isomer of triiodothyronine (T3), a form of thyroid hormone used to treat hypothyroidism and myxedema coma. It is marketed under the brand name Cytomel (or Tertroxin in Australia).

T3 is metabolically active hormone that is produced from T4. T4 is deiodinated by two deiodinases to produce the active triiodothyronine: 1. Type I present within the liver and accounts for 80% of the deiodination of T4 2. Type II present within the pituitary. T4 is synthesised in the thyroid gland follicular cells as follows. 1. The Na+/I- symporter transports two sodium ions across the basement membrane of the follicular cells along with an iodine ion. This is secondary active transporter that utilises the concentration gradient of Na+ to move I- against its concentration gradient. 2. I- is moved across the apical membranae into the

The thyroid hormones, thyroxine (T4) and triiodothyronine (T3), are tyrosine-based hormones produced by the thyroid gland. An important component in the synthesis of thyroid hormones is iodine. The major form of thyroid hormone in the blood is thyroxine (T4), which has a longer half life than T3. The ratio of T4 to T3 released in the blood is roughly 20 to 1. Thyroxine is converted to the active T3 (three to four times more potent than T4) within cells by deiodinases (5'-iodinase). These are further processed by decarboxylation and deiodination to produce iodothyronamine (T1a) and thyronamine (T0a).

T3 and T4 are carried in the blood bound to plasma proteins. This has the effect of increasing the half life of the hormone and decreasing the rate at which it is taken up by peripheral tissues. There are three main proteins that the two hormones are bound to. Thyronine binding globulin (TBG) is a gylcoprotein that has a higher affinity for T4 than for T3. The second plasma protein to which the hormone bind is transthyretin (which has a higher affinity for T3 than for T4). Both hormones bind with a low affinity to serum albumin, but due to

A doctor may add a medication with a different mode of action to bolster the effect of an antidepressant in cases of treatment resistance.[145] Medication with lithium salts has been used to augment antidepressant therapy in those who have failed to respond to antidepressants alone.[146] Furthermore, lithium dramatically decreases the suicide risk in recurrent depression.[147] Addition of a thyroid hormone, triiodothyronine may work as well as lithium, even in patients with normal thyroid function.[148] Addition of atypical antipsychotics when the patient has not responded to an antidepressant is also known to increase the effectiveness of antidepressant drugs, albeit offset by

Both excess and deficiency of thyroxine can cause disorders. Hyperthyroidism (an example is Graves Disease) is the clinical syndrome caused by an excess of circulating free thyroxine, free triiodothyronine, or both. It is a common disorder that affects approximately 2% of women and 0.2% of men. Thyrotoxicosis is often used interchangeably with hyperthyroidism, but there are subtle differences. Although thyrotoxicosis also refers to an increase in circulating thyroid hormones, it can be caused by the intake of thyroxine tablets or by an over-active thyroid, whereas hyperthyroidism refers solely to an over-active thyroid. Hypothyroidism (an example is Hashimoto's thyroiditis) is the case where

Thyroxine, or 3,5,3',5'-tetraiodothyronine (often abbreviated as T4), a form of thyroid hormones is the major hormone secreted by the follicular cells of the thyroid gland. Thyroxine is synthesized via the iodination and covalent bonding of the phenyl portions of tyrosine residues found in an initial peptide, thyroglobulin, which is secreted into thyroid granules. These iodinated diphenyl compounds are cleaved from their peptide backbone upon being stimulated by thyroid stimulating hormone. More in the T3 and T4 section of thyroid. T4 is transported in blood, with 99.95% of the secreted T4 being protein bound, principally to thyroxine-binding globulin (TBG), and, to a

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