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In the bloodstream, less than one percent of the circulating T 3 and T 4 remains unbound. This free T 3 and T 4 can cross the lipid bilayer of cell membranes and be taken up by cells. The remaining 99 percent of circulating T 3 and T 4 is bound to specialized transport proteins called thyroxine-binding globulins (TBGs), to albumin, or to other plasma proteins. This “packaging” prevents their free diffusion into body cells. When blood levels of T 3 and T 4 begin to decline, bound T 3 and T 4 are released from these plasma proteins and readily cross the membrane of target cells. T 3 is more potent than T 4 , and many cells convert T 4 to T 3 through the removal of an iodine atom.

Regulation of th synthesis

The release of T 3 and T 4 from the thyroid gland is regulated by thyroid-stimulating hormone (TSH). As shown in [link] , low blood levels of T 3 and T 4 stimulate the release of thyrotropin-releasing hormone (TRH) from the hypothalamus, which triggers secretion of TSH from the anterior pituitary. In turn, TSH stimulates the thyroid gland to secrete T 3 and T 4 . The levels of TRH, TSH, T 3 , and T 4 are regulated by a negative feedback system in which increasing levels of T 3 and T 4 decrease the production and secretion of TSH.

Classic negative feedback loop

This diagram illustrates a negative feedback loop. It shows the general steps of a negative feedback loop at the center (imbalance, hormone release, correction, and negative feedback) using the example of the hormone cascade that regulates metabolic rate. The hypothalamus releases TRH in response to low metabolic rate and or low T three and T four concentrations in the blood (imbalance). This triggers TSH release by the pituitary (hormone release). The TSH travels to the thyroid where it triggers T three and T four release by the thyroid cells. T three and T four increase basal metabolic rate of the body cells and cause a rise in body temperature (the calorigenic effect). T three and T four then feed back to the hypothalamus and inhibits TRH and TSH release. If metabolic rate is high and or T three and T four concentrations are low, then the hypothalamus stops releasing TRH (negative feedback). As a result, the anterior pituitary will not release TSH and no T three or T four will be produced by the thyroid.
A classic negative feedback loop controls the regulation of thyroid hormone levels.

Functions of thyroid hormones

The thyroid hormones, T 3 and T 4 , are often referred to as metabolic hormones because their levels influence the body’s basal metabolic rate, the amount of energy used by the body at rest. When T 3 and T 4 bind to intracellular receptors located on the mitochondria, they cause an increase in nutrient breakdown and the use of oxygen to produce ATP. In addition, T 3 and T 4 initiate the transcription of genes involved in glucose oxidation. Although these mechanisms prompt cells to produce more ATP, the process is inefficient, and an abnormally increased level of heat is released as a byproduct of these reactions. This so-called calorigenic effect (calor- = “heat”) raises body temperature.

Adequate levels of thyroid hormones are also required for protein synthesis and for fetal and childhood tissue development and growth. They are especially critical for normal development of the nervous system both in utero and in early childhood, and they continue to support neurological function in adults. As noted earlier, these thyroid hormones have a complex interrelationship with reproductive hormones, and deficiencies can influence libido, fertility, and other aspects of reproductive function. Finally, thyroid hormones increase the body’s sensitivity to catecholamines (epinephrine and norepinephrine) from the adrenal medulla by upregulation of receptors in the blood vessels. When levels of T 3 and T 4 hormones are excessive, this effect accelerates the heart rate, strengthens the heartbeat, and increases blood pressure. Because thyroid hormones regulate metabolism, heat production, protein synthesis, and many other body functions, thyroid disorders can have severe and widespread consequences.

Disorders of the…

Endocrine system: iodine deficiency, hypothyroidism, and hyperthyroidism

As discussed above, dietary iodine is required for the synthesis of T 3 and T 4 . But for much of the world’s population, foods do not provide adequate levels of this mineral, because the amount varies according to the level in the soil in which the food was grown, as well as the irrigation and fertilizers used. Marine fish and shrimp tend to have high levels because they concentrate iodine from seawater, but many people in landlocked regions lack access to seafood. Thus, the primary source of dietary iodine in many countries is iodized salt. Fortification of salt with iodine began in the United States in 1924, and international efforts to iodize salt in the world’s poorest nations continue today.

Dietary iodine deficiency can result in the impaired ability to synthesize T 3 and T 4 , leading to a variety of severe disorders. When T 3 and T 4 cannot be produced, TSH is secreted in increasing amounts. As a result of this hyperstimulation, thyroglobulin accumulates in the thyroid gland follicles, increasing their deposits of colloid. The accumulation of colloid increases the overall size of the thyroid gland, a condition called a goiter    ( [link] ). A goiter is only a visible indication of the deficiency. Other iodine deficiency disorders include impaired growth and development, decreased fertility, and prenatal and infant death. Moreover, iodine deficiency is the primary cause of preventable mental retardation worldwide. Neonatal hypothyroidism (cretinism) is characterized by cognitive deficits, short stature, and sometimes deafness and muteness in children and adults born to mothers who were iodine-deficient during pregnancy.

Goiter

This photo shows a woman with a goiter, which is an extreme, irregular swelling on the anterior side of the neck.
(credit: “Almazi”/Wikimedia Commons)

In areas of the world with access to iodized salt, dietary deficiency is rare. Instead, inflammation of the thyroid gland is the more common cause of low blood levels of thyroid hormones. Called hypothyroidism    , the condition is characterized by a low metabolic rate, weight gain, cold extremities, constipation, reduced libido, menstrual irregularities, and reduced mental activity. In contrast, hyperthyroidism    —an abnormally elevated blood level of thyroid hormones—is often caused by a pituitary or thyroid tumor. In Graves’ disease, the hyperthyroid state results from an autoimmune reaction in which antibodies overstimulate the follicle cells of the thyroid gland. Hyperthyroidism can lead to an increased metabolic rate, excessive body heat and sweating, diarrhea, weight loss, tremors, and increased heart rate. The person’s eyes may bulge (called exophthalmos) as antibodies produce inflammation in the soft tissues of the orbits. The person may also develop a goiter.

Calcitonin

The thyroid gland also secretes a hormone called calcitonin    that is produced by the parafollicular cells (also called C cells) that stud the tissue between distinct follicles. Calcitonin is released in response to a rise in blood calcium levels. It appears to have a function in decreasing blood calcium concentrations by:

  • Inhibiting the activity of osteoclasts, bone cells that release calcium into the circulation by degrading bone matrix
  • Increasing osteoblastic activity
  • Decreasing calcium absorption in the intestines
  • Increasing calcium loss in the urine

However, these functions are usually not significant in maintaining calcium homeostasis, so the importance of calcitonin is not entirely understood. Pharmaceutical preparations of calcitonin are sometimes prescribed to reduce osteoclast activity in people with osteoporosis and to reduce the degradation of cartilage in people with osteoarthritis. The hormones secreted by thyroid are summarized in [link] .

Thyroid Hormones
Associated hormones Chemical class Effect
Thyroxine (T 4 ), triiodothyronine (T 3 ) Amine Stimulate basal metabolic rate
Calcitonin Peptide Reduces blood Ca 2+ levels

Of course, calcium is critical for many other biological processes. It is a second messenger in many signaling pathways, and is essential for muscle contraction, nerve impulse transmission, and blood clotting. Given these roles, it is not surprising that blood calcium levels are tightly regulated by the endocrine system. The organs involved in the regulation are the parathyroid glands.

Chapter review

The thyroid gland is a butterfly-shaped organ located in the neck anterior to the trachea. Its hormones regulate basal metabolism, oxygen use, nutrient metabolism, the production of ATP, and calcium homeostasis. They also contribute to protein synthesis and the normal growth and development of body tissues, including maturation of the nervous system, and they increase the body’s sensitivity to catecholamines. The thyroid hormones triiodothyronine (T 3 ) and thyroxine (T 4 ) are produced and secreted by the thyroid gland in response to thyroid-stimulating hormone (TSH) from the anterior pituitary. Synthesis of the amino acid–derived T 3 and T 4 hormones requires iodine. Insufficient amounts of iodine in the diet can lead to goiter, cretinism, and many other disorders.

Questions & Answers

master gland Kon si h
kajal Reply
Pituitary gland
KUNDAN
pituitary gland because it give harmonies and control other gland
ABDULLAH
glands often secrete hormones which play an important role in maintaining homeostasis.
KUNDAN
pituitary gland
Aftab
pituitary gland is master gland because it present in biran and secrtehormones and play other gland of human body and function its gland so that also called master gland.
ABDULLAH
pituitary glandis the master gland of our body
Nadeem
how do you study for A&P? lab and lecture?
Aubrey Reply
When one is suffering from motion sickness what area of the brain may trigger emesis?
amy Reply
that is the sign of stroke. if the patient have a stroke. the left side of body is weakness, the affected right of cerebrum but if right side of the body is weakness surely in left side!
Larry
thank you sooo much bro
Fatima Reply
helloo
Sentamu
Fatima hw a u
Sentamu
hii
Shubham
any one elaborate fr me foramens of the skull and features which they transmit
Sentamu
icant undrestand plz
zahruuzh Reply
try to read I hop you will understand
state and explain 20 radiology uses
ILYAS Reply
what are chemicals in anatomy and physiology?
Mike Reply
what can I do to find it easy for me in anatomy and physiology course
Mike
study up on the basics of the periodic chart, learn bones and muscles attachments, and learn muscles. Those take the longest to memorize. After that it should be a little easier.
Toni
what are the two types of body cells
Jennifer Reply
what is malnutrition
Claire
malnutrition refers to faulty nutrition resulting from malabsorption,poor diet or overeating. Sometimes too these food do not contain all the six food nutrients in their right proportion.
Yakubu
thank you
Claire
welcome
Yakubu
Will u be malnourished?
Baigwa
gud
Budumari
What's the difference between radiology and radiology
gabriel
Nothing! Radiology it means the study or using of radiation in medical science it can be 1.diagnose or treatment diagnosed radiology! x- ray. ultrasound. ct-scan. mammogram. MRI. 2. treatmen- radiation oncology, like Cobalt 60. and nuclear medicine
Larry
what is X-ray?
Nissar
X-ray is type of light that make it possible to see inside any object. as human body
KUNDAN
How the nervous system develops
ayiesher Reply
From the cells at the back of an embryo
Mma
breifly explain anatomy of thorax
Hadiza Reply
How many region the rib is divided
Konneh
how many bon of human being
Nura
206
John
how to study for the skeletal system
ryaisha
and anatomy
ryaisha
I really need sources immediately
ryaisha
I printed out all the different bones. Put them in the see through protective sheaths and got dry erase markers. I could right on them and erase to help me learn to spell the names of markings and bones.
Toni
Or go to a book store, Barnes and Noble(doesn't have to be this) and they have coloring books for anatomy. $16. Really helpful.
Toni
okay thanks and are what study tools you use to study the materials and get a better understanding
ryaisha
*what are
ryaisha
hiii
Roopa
I prefer diagrams, pictures that lay out each step with the information in each step. For Example: how action potential creates muscles to move. A pictured diagram gives me a better understanding of how each piece plays a role in each step of the process.
Toni
Also for basics, such as memorizing vocab. Flash cards are great. Don't become discouraged if you don't get them all right the first times through. The more you go through them, your brain will remember pieces of information from each and help you to pull out the information 😉
Toni
okay thanks
ryaisha
what is the functions of the lips in human
Momboi Reply
could I say sensation?
Sovilace
kissing
Obrian
for protection
Omar
Lips assist in speech and eating
Cindy
Too many easy questions. Which bones are the axial and appendicular? What are the abbreviations for TEE, TTE, AED, A-Mode, B-Mode, and LTH? What is the difference between hypothalamus and thalamus? Where is the parathyroid located? How many True Ribs do we have?
Sovilace Reply
Transesophageal echocardiography Transthoracic Echocardiography automated external defibrillator brightnees mode Los Hermanos Taverna
Inam
what is anatomy
okello Reply
anatomy is scientific study of body structures and how they relate to each other
Skeater
Are there other functions of the nucleolus apart from synthesis of RNA and formation of ribosomes
Peninah Reply
plays a role in cell response to stress
airiz
what is angle of auscultation
Bryan Reply

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Source:  OpenStax, Anatomy & Physiology. OpenStax CNX. Feb 04, 2016 Download for free at http://legacy.cnx.org/content/col11496/1.8
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