The thyroxine gland?

Question

Im doing a project on the thyroxine hormone found in the thyroid gland. Can somone give me some information or a really good website of the feedback mechanisms in this hormone?

Answer 1

OK, let's do this. We are concerned with the pituitary gland & the thyroid gland. The thyroid gland produces thyroxin, AKA - t3 & t4 or circulating thyroid hormone. The pituitary gland produces TSH or thyroid stimulating hormone. TSH from the pituitary stimulates the thyroid to release more thyroxin into the circulation. As the level of thyroxin reaches needed levels, it turns off the release of TSH from the pituitary by feedback inhibition. To review, as TSH increases - thyroxin release increases./ As thyroxin levels reach adequate levels, it decreases release of TSH. Let's use that information to see if you really understand........If TSH levels are very high & thyroxin levels are low....the problem is in the thyroid & is highly suggestive of hypothyroidism.........If TSH levels are low & thyroxin levels are very high....the problem is in the thyroid & is highly suggestive of hyperthyroidism......If TSH & thyroxin levels are low, this is suggestive of a problem in the pituitary gland. When TSH & thyroxin levels are as they should be, this is normal & is called euthyroidism. I hope this answers your question.

Answer 2

1

Answer 3

The thyroid gland is butterfly shaped and lies on either side of the larynx, just below the thyroid cartilage.
The secretion of the thyroid hormones T4 and T3 is controlled by the thyroid stimulation hormone TSH secreted by the anterior lobe of the pituitary gland.
The bodys ability to secrete thyroid hormones depends on the uptake of iodine from food and water.
The primary function of the thyroid gland is to regulate the bodys metabolism. which is the rate the body uses energy and the speed of which body functions work.
The thyroid secretions also influence growth and the functioning of the nervous system.
The primary thyroid hormones are THYROXINE T4 and triiodothyronine T3
Calcitonin also known as thyrocalcitonin works with the parathyroid hormone to regulate calcium levels in the blood and tissues. Calcitonin decreases blood levels by moving calcium into storage in the bones and teeth. Compare with PTH
Thyroid cancer is commonly first indicated by an enlargement of the thyroid gland. with early detection and treatment the survival rate is high. without treatment the cancer will spread to other parts of the body.
Hypothyroidism also known as an underactive thyroid is a deficiency of thyroid secretion
Cretinism is a congenital lack of thyroid secretion. If treatment is not started soon after birth it causes arrested physical and mental development
Myxedema is a severe form of adult hypothyroidism. Symptoms include an enlarged tongue and puffiness of the hands and face.
Hashimotos thyroiditis is an autoimmune disorder in which the immune system mistakenly attacks thyroid tissue, setting up an inflammatory process that may progressively destroy the gland. THis may cause goiter or hypothyroidism
Hyperthyroidism is a condition of excessive thyroid hormones in the blood.
Graves disease is an autoimmune disorder characterized by hyperthyroidism, goiiter and exphthalmos
Goiter is an abnormal enlargement of the thyroid gland that produces a swelling in the front part of the neck.
Exophthalmos is an abnormal protrusion of the eyes.
Thrytoxicosis also known as thyroid storm is a life threatening condition resulting from the release of excessive quantities of the thyroid hormones into the bloodstream
A thyroid scan is one means to measure thyroid function
AN antithyroid drug is a medication administered to slow the ability of the thyroid gland to produce thyroid hormones
A chemical thyroidectomy . also known as radioactive iodine therapy is the administration of radioactive iodine to destory thyroid cells. THis is used to treat ;hyperthyroid disorders such as Graves disease
A lobectomy is the removal of one lobe of the thyroid gland. This term is also used to describe the removal of a lobe of the liver. brain, or lung
A thyroid stimulating hormone assay is a diagnostic test to measure circulating blood levels of TSH. The test is used to detect abnormal thyroid activity resulting from excessive pituitary stimulation.

THis is all I can tell you about anything to do with the thyroid gland and its functions

Answer 4

Thyroxine is a hormone that is naturally produced by the thyroid gland. There is no health risk connected to taking the correct dose.

Answer 5

thyroxine gland

Answer 6

http://www.besthealth.com/besthealth/bodyguide/reftext/html/endo_sys_fin.html#homeostatic

Answer 7

The thyroid (from the Greek word for "shield", after its shape) is one of the larger endocrine glands in the body. It is a double-lobed structure located in the neck and produces hormones, principally thyroxine (T4) and triiodothyronine (T3), that regulate the rate of metabolism and affect the growth and rate of function of many other systems in the body. The hormone calcitonin is also produced and controls calcium blood levels. Iodine is necessary for the production of both hormones. Hyperthyroidism (overactive thyroid) and hypothyroidism (underactive thyroid) are the most common problems of the thyroid gland. Specialists are called Thyroidologists.

Anatomy

The thyroid is situated on the front side of the neck, starting at the oblique line on the thyroid cartilage (just below the laryngeal prominence or Adam's apple), and extending to the 6th Tracheal ring (C-shaped cartilagenous ring of the trachea). Vertebral levels are inappropriate to demarcate the gland's upper and lower border with vertebral levels as it moves position in relation to these during swallowing. It lies over the trachea and is covered by layers of pretracheal fascia (allowing it to move), muscle and skin.

The thyroid is one of the larger endocrine glands - 10-20 grams in adults and butterfly-shaped. The wings correspond to the lobes and the body to the isthmus of the thyroid. It may enlarge substantially during pregnancy and when affected by a variety of diseases.

[edit] Blood supply
Blood supply
Blood supply

The thyroid gland is supplied by two pairs of arteries: the superior and inferior thyroid arteries of each side. The superior thyroid artery is the first branch of the external carotid, and supplies mostly the upper half of the thyroid gland, while the inferior thyroid artery is the major branch of the thyrocervical trunk, which comes off of the subclavian artery. In 10% of people, there is an additional thyroid artery, the thyreoidea ima, that arises from the brachiocephalic trunk or the arch of the aorta. Lymph drainage follows the arterial supply.

There are three main veins that drain the thyroid to the superior vena cava: the superior, middle and inferior thyroid veins.

In comparison to the other organs of the body, the Thyroid receives one of the largest blood supplies per gram weight.[citation needed] The largest blood supply is seen in the Carotid arch baroreceptor organ.[citation needed]

[edit] Embryologic development
Floor of pharynx of embryo between 18 and 21 days.
Floor of pharynx of embryo between 18 and 21 days.

The thyroid is derived from the second pharyngeal arch. In the fetus, at 3-4 weeks of gestation, the thyroid gland appears as an epithelial proliferation in the floor of the pharynx at the base of the tongue between the tuberculum impar and the copula at a point latter indicated by the foramen cecum. Subsequently the thyroid descends in front of the pharyngeal gut as a bilobed diverticulum through the thyroglossal duct. Over the next few weeks, it migrates to the base of the neck. During migration, the thyroid remains connected to the tongue by a narrow canal, the thyroglossal duct.

Follicles of the thyroid begin to make colloid in the 11th week and thyroxine by the 18th week.

[edit] Histology of the thyroid

At a histological level, there are three primary features of the thyroid:
Feature Description
follicles The thyroid is composed of spherical follicles that selectively absorb iodine (as iodide ions, I-) from the blood for production of thyroid hormones. Twenty-five percent of all the body's iodide ions are in the thyroid gland. Inside the follicles is a colloid which is rich in a protein called thyroglobulin. The colloidal material serves as a reservoir of materials for thyroid hormone production and, to a lesser extent, a reservoir of the hormones themselves.
thyroid epithelial cells (or "follicular cells") The follicles are surrounded by a single layer of thyroid epithelial cells, which secrete T3 and T4.
parafollicular cells (or "C cells" Scattered among follicular cells and in spaces between the spherical follicles are another type of thyroid cell, parafollicular cells, which secrete calcitonin.

[edit] Physiology

The primary function of the thyroid is production of the hormones thyroxine (T4), triiodothyronine (T3), and calcitonin. Up to 80% of the T4 is converted to T3 by peripheral organs such as the liver, kidney and spleen. T3 is about ten times more active than T4[1].

[edit] T3 and T4 production and action

Thyroxine is synthesised by the follicular cells from free tyrosine and on the tyrosine residues of the protein called thyroglobulin (TG). Iodine, captured with the "iodine trap" by the hydrogen peroxide generated by the enzyme thyroid peroxidase (TPO)[2] and linked to the 3' and 5' sites of the benzene ring of the tyrosine residues on TG, and on free tyrosine. Upon stimulation by TSH (see below), the follicular cells reabsorb TG and proteolytically cleave the iodinated tyrosines from TG, forming T4 and T3 (in T3, one iodine is absent compared to T4), and releasing them into the blood. Deiodinase enzymes convert T4 to T3[3]. Thyroid hormone that is secreted from the gland is about 90% T4 and about 10% T3[1].

Cells of the brain are a major target for thyroid hormone. Thyroid hormones play a particularly crucial role in brain development during pregnancy[4]. A transport protein (OATP1C1) has been identified that seems to be important for T4 transport across the blood brain barrier[5]. A second transport protein (MCT8) is important for T3 transport across brain cell membranes[5].

In the blood, T4 and T3 are partially bound to thyroxine-binding globulin, transthyretin and albumin. Only a very small fraction of the circulating hormone is free (unbound) - T4 0.03% and T3 0.3%. Only the free fraction has hormonal activity. As with the steroid hormones and retinoic acid, thyroid hormones cross the cell membrane and bind to intracellular receptors (α1, α2, β1 and β2), which act alone, in pairs or together with the retinoid X-receptor as transcription factors to modulate DNA transcription[1].

[edit] T3 and T4 regulation

The production of thyroxine is regulated by thyroid-stimulating hormone (TSH), released by the anterior pituitary. The thyroid and thyrotropes form a negative feedback loop: TSH production is suppressed when the T4 levels are high, and vice versa. The TSH production itself is modulated by thyrotropin-releasing hormone, which is produced by the hypothalamus and secreted at an increased rate in situations such as cold (in which an accelerated metabolism would generate more heat). TSH production is blunted by somatostatin (SRIH), rising levels of glucocorticoids and sex hormones (estrogen and testosterone), and excessively high blood iodide concentration.

[edit] Calcitonin

An additional hormone produced by the thyroid contributes to the regulation of blood calcium levels. Parafollicular cells produce calcitonin in response to hypercalcemia. Calcitonin stimulates movement of calcium into bone, in opposition to the effects of parathyroid hormone. However calcitonin seems far less essential than PTH, as calcium metabolism remains clinically normal after removal of the thyroid, but not the parathyroids.

It may be used diagnostically as a tumor marker for a form of thyroid cancer (medullary thyroid adenocarcinoma), in which high calcitonin levels may be present and elevated levels after surgery may indicate recurrence. It may even be used on biopsy samples from suspicious lesions (e.g. swollen lymph nodes) to establish whether they are metastasis of the original cancer.

Calcitonin can be used therapeutically for the treatment of hypercalcemia or osteoporosis.

[edit] The significance of iodine

In areas of the world where iodine (essential for the production of thyroxine, which contains four iodine atoms) is lacking in the diet, the thyroid gland can be considerably enlarged, resulting in the swollen necks of endemic goitre.

Thyroxine is critical to the regulation of metabolism and growth throughout the animal kingdom. Among amphibians, for example, administering a thyroid-blocking agent such as propylthiouracil (PTU) can prevent tadpoles from metamorphosing into frogs; conversely, administering thyroxine will trigger metamorphosis.

In humans, children born with thyroid hormone deficiency will have physical growth and development problems, and brain development can also be severely impaired, in the condition referred to as cretinism. Newborn children in many developed countries are now routinely tested for thyroid hormone deficiency as part of newborn screening by analysis of a drop of blood. Children with thyroid hormone deficiency are treated by supplementation with synthetic thyroxine, which enables them to grow and develop normally.

Because of the thyroid's selective uptake and concentration of what is a fairly rare element, it is sensitive to the effects of various radioactive isotopes of iodine produced by nuclear fission. In the event of large accidental releases of such material into the environment, the uptake of radioactive iodine isotopes by the thyroid can, in theory, be blocked by saturating the uptake mechanism with a large surplus of non-radioactive iodine, taken in the form of potassium iodide tablets. While biological researchers making compounds labelled with iodine isotopes do this, in the wider world such preventive measures are usually not stockpiled before an accident, nor are they distributed adequately afterward. One consequence of the Chernobyl disaster was an increase in thyroid cancers in children in the years following the accident. [2]

The use of iodised salt is an efficient way to add iodine to the diet. It has eliminated endemic cretinism in most developed countries, and some governments have made the iodination of flour mandatory. Potassium iodide and Sodium iodide are the most active forms of supplemental iodine.

[edit] Diseases of the thyroid gland

Hyper- and hypofunction (affects about 2% of the population):

* Hypothyroidism (underactivity)
o Hashimoto's thyroiditis / thyroiditis
o Ord's thyroiditis
o Postoperative hypothyroidism
o Postpartum thyroiditis
o Silent thyroiditis
o Acute thyroiditis
o Iatrogenic hypothyroidism
* Hyperthyroidism (overactivity)
o Thyroid storm
o Graves-Basedow disease
o Toxic thyroid nodule
o Toxic nodular struma (Plummer's disease)
o Hashitoxicosis
o Iatrogenic hyperthyroidism
o De Quervain thyroiditis (inflammation starting as hyperthyroidism, can end as hypothyroidism)

Anatomical problems:

* Goitre
o Endemic goitre
o Diffuse goitre
o Multinodular goitre
* Lingual thyroid
* Thryoglossal duct cyst

Tumors:

* Thyroid adenoma
* Thyroid cancer
o Papillary
o Follicular
o Medullary
o Anaplastic
* Lymphomas and metastasis from elsewhere (rare)

Deficiencies:

* Cretinism

Medication linked to thyroid disease includes amiodarone, lithium salts, some types of interferon and IL-2.

[edit] Diagnosis

[edit] Blood tests

* The measurement of thyroid-stimulating hormone (TSH) levels is often used by doctors as a screening test. Elevated TSH levels can signify an inadequate hormone production, while suppressed levels can point at excessive unregulated production of hormone.

* If TSH is abnormal, decreased levels of thyroid hormones T4 and T3 may be present; these may be determined to confirm this.

* Autoantibodies may be detected in various disease states (anti-TG, anti-TPO, TSH receptor stimulating antibodies).

* There are two cancer markers for thyroid derived cancers. Thyroglobulin (TG) for well differentiated papillary or follcular adenocarcinoma, and the rare medullary thyroid cancer has calcitonin as the marker.

* Very infrequently, TBG and transthyretin levels may be abnormal; these are not routinely tested.

[edit] Ultrasound

Nodules of the thyroid may require medical ultrasonography to establish their nature.

The main characteristics of a thyroid nodule on high frequency thyroid ultrasound are as follows:
Possible cancer Benign characteristics
irregular border smooth borders
hypoechoic (less echogenic than the surrounding tissue) hyperechoic
microcalcifications -
taller than wide shape on transverse study -
significant intranodular blood flow by power Doppler -
- "comet tail" artifact as sound waves bounce off intranodular colloid

However, these criteria alone can help select nodules for biopsy, but no criterion is 100%.

[edit] Biopsy

The ideal way to assure that a nodule is not cancerous is a biopsy. To be sure you have sampled the specific nodule of interest, even if you can not feel it, ultrasound guided fine needle aspiration is recommended. Free hand fine needle aspiration (FNA) may be performed, on palpable nodules, but has a higher error rate, or inadequate sample result.

If a result is not conclusive, thyroid scintigraphy with iodine-123 may reveal whether the nodule is abnormally active ("hot") or inactive ("cold"). Hot nodules are very, very rarely cancerous, therefore the endocrinologist may not need to repeat the biopsy. However if it is not hot, an inconclusive FNA result may warrant a repeat biopsy, but this time, not by free hand, but by ultrasound guided FNA technique.

[edit] Special concerns about Hashimoto's thyroiditis

Hashimoto's thyroiditis may be the background disease for a rapidly growing mass caused by a rare case of thyroid lymphoma. The rapid growth in a previous long standing stable thyroiditis should be biopsied by ultrasound guided needle, and live cells collected in special medium for flow cytometry. The exact type of lymphoma can be determined by FNA, without surgery.

[edit] Treatment

[edit] Medical treatment

Levothyroxine is a stereoisomer of thyroxine which is degraded much slower and can be administered once daily in patients with hypothyroidism.

Graves' disease may be treated with the thioamide drugs propylthiouracil, carbimazole or methimazole, or rarely with Lugol's solution. Hyperthyroidism as well as thyroid tumors may be treated with radioactive iodine.

Percutaneous Ethanol Injections, PEI, for therapy of recurrent thyroid cysts, and metastatic thyroid cancer lymph nodes, as an alternative to the usual surgical method.

[edit] Thyroid surgery

Thyroid surgery is performed for a variety of reasons. A nodule or lobe of the thyroid is sometimes removed for biopsy or for the presence of an autonomously functioning adenoma causing hyperthyroidism. A large majority of the thyroid may be removed, a subtotal thyroidectomy, to treat the hyperthyroidism of Graves' disease, or to remove a goitre that is unsightly or impinges on vital structures. A complete thyroidectomy of the entire thyroid, including associated lymph nodes, is the preferred treatment for thyroid cancer. Removal of the bulk of the thyroid gland usually produces hypothyroidism, unless the person takes thyroid hormone replacement.

If the thyroid gland must be removed surgically, care must be taken to avoid damage to adjacent structures, the parathyroid glands and the recurrent laryngeal nerve. Both are susceptible to accidental removal and/or injury during thyroid surgery. The parathyroid glands produce parathyroid hormone (PTH), a hormone needed to maintain adequate amounts of calcium in the blood. Removal results in hypoparathyroidism and a need for supplemental calcium and vitamin D each day. The recurrent laryngeal nerves provide motor control for all external muscles of the larynx except for the cricothyroid muscle, also runs along the posterior thyroid. Accidental laceration of either of the two or both recurrent laryngeal nerves may cause paralysis of the vocal cords and their associated muscles, changing the voice quality.

[edit] Radioiodine therapy

Large goiters that cause symptoms, but do not harbor cancer, after evaluation, and biopsy of suspicious nodules can be treated by an alternative therapy with radioiodine. The iodine uptake can be high in countries with iodine deficiency, but low in iodine sufficient countries. The 1999 release of rhTSH thyrogen in the USA, can boost the uptakes to 50-60% allowing the therapy with iodine 131. The gland shrinks by 50-60%, but can cause hypothyroidism, and rarely pain syndrome cause by radiation thyroiditis that is short lived and treated by steroids. Rare cases of Graves' disease have been reported after goiter I/131 therapy. This is still an off label use of Thyrogen, but is a very excellent alternative to surgery.