Thyroid is butterfly shaped and an endocrine gland secreted thyroid hormones which regulated rate of metabolism of human body. These hormones are essential for human. The gland produces two types of hormones. Major hormone is called “thyroxin(T4)”, the other one is T3(triiodotironin). T4 is produced by follicular cells. T4 is inactive form of thyroid hormone. Periferal body cells like tissues and muscles convert T4 to T3 which active form. Minor amounts of T3 are also produced by follicular cell of thyroid gland.
Thyroid gland has also another type of cell which called parafollicular. These cells produce calcitonin hormone which regulated calcium in blood. I won’t mention about this mechanism in this section.
Anatomy
Thyroid gland is located front of trachea and below thyroid cartilage. It weights about 15-30 grams. It has three parts like lateral lobes and isthmus. Thyroid gland originates from pharingeal pouches(root of tongue) and migrates to anterior of trachea in embryonic life. Sometimes this development interrupts and the gland is located abnormal area. This situation describes as ectopic thyroid gland.
Physiology
Thyroid hormones originates from tyrosin aminoacid. Iodine element is taken by sodium-iodine symporter protein(NIS) to inside of follicular cells. Then the iodine is oxidized by TPO(thyroperoxidase) enzymes. Tyrosin is coupled with iodine as monoiodotyrosin(MIT) and diiodotyrosin(DIT). T3(triiodotironin) consists of one MIT an one DIT residue. T4(thyroxin) is generated by combining two DIT residues.
Thyroid hormones are stored in tyroglobulin. Tyroglobulin is a protein which consists of large amount tyrosin aminoacids. It is located in lumen of follicular cells. Thyroid hormones syntesis is regulated by Thyroid Stimulating Hormone(TSH) which secreted by pituitary gland. TSH initiates iodine uptake, organification, coupling and releasing thyroid hormones.
Effects of Thyroid Hormones
Thyroid hormones regulate vital body functions;
- Increases synthesis of proteins, enzymes and hormones in cells.
- Increases metabolic activity of cells.
- Increases DNA and RNA synthesis in cells.
- Activates fat tissue to release fatty acid.
- Manages heart rate and blood pressure.
- Coordinates normal growth of body in puberte.
- Increases muscle contractility.
- Increases bone mineralization.
- It is necessary for brain development of fetus.
- Manages gastrointestinal motility and glucose absorption.
- Regulates cholesterol levels.
- Manages menstrual cycle.
and much more..
Functional Disorders of Thyroid Gland
Generally, thyroid malfunctions split to two types:
1) Hypothyroidism:
It is described as decreased thyroid hormone levels in blood. Patients have some symptoms such as;
- Bradycardia(low heart rate),
- Tiredness,
- Weight gain,
- Mental slowing,
- Dry and hair loss.
- Sleepiness.
- Constipation
Hypothyroidism is a clinical syndrome caused by insufficient hormone synthesis or destruction of thyroid gland. TSH level decreases in blood. T4 and T3 levels can be normal or decreased.
Etiology of Hypothyroidism
Insufficient uptake iodine causes decreased production of thyroid hormone. The gland enlarges to produce much more hormones. This is called goiter. There may be insufficient iodine feeding behavior in some people who live endemic areas.
Some viruses can destruct the thyroid gland and cause reduced production of hormones. This is called thyroiditis. Hypothyroidism is seen in late period of thyroiditis
Hashimoto thyroiditis is an autoimmun disease caused by self-destruction of some proteins in thyroid gland by antibodies. Especially women have this disease. Thyroid gland becomes smaller depending on destruction.
Treatment of Hypothyroidism
Hypothyroidism is treated by replacement of thyroid hormone drugs.
2) Hyperthyroidism:
It is a clinical syndrome that increased thyroid hormone more than physiological demands in blood. Excessive hormones can have dangerous effects on body. Patients may complaint from these symptoms;
- Tachycardia and palpitation
- Weight loss despite increased appetite
- Heat intolerance
- Tremor on hands
- Nervousness and irritability
- Atrial fibrilation
- Increased gastrointestinal motility and diarrhea.
- Exopthalmos
Excessive thyroid hormones are also called thyrotoxicosis. TSH is supressed and increased T3 and T4 levels are detected in blood. They have toxic effects on cardiac muscles. Therefore hyperthyroidism is an important clinical problem and it requires immediate treatment.
Etiology of Hyperthyroidism
Graves Disease is the most important reason of hyperthyroidism. It is the similar to Hashimoto Thyroiditis but there is not destruction of the gland. Some autoimmune antibodies stimulate TSH receptors and all production mechanisms of thyroid hormones accelerate. Excessive thyroid hormones suppress TSH levels and increase whole body rate of metabolism.
Subacute Thyroiditis can cause hyperthyroidism because of releasing hormone caused by destruction during early period. Viruses role this clinic. Thyroid gland is sensitive and painful. Thyroid scintigraphy helps to diagnose highly accuracy.
Thyroid gland can have nodules that produced hormone autonomously. They are called toxic adenoma. The underlying mechanisms have not been able to described clearly yet.
Treatment of Hyperthyroidism
There are three treatment options:
1) Antithyroidal drugs are used for initial treatment by blockaging the synthesis of thyroid hormones. Patient should take these drugs for two years. If disease can not be controlled because of advers effects or insufficiency the other options are considered.
2) Thyroidectomy: Uncontrolled hyperthyroidism with drugs can be treated with surgical operation which called thyroidectomy. It is performed with an 5 cm incision on neck. Thyroid gland is removed totally. However, some complications can be occur such as hoarseness and hypocalcemia.
3) RAI Treatment: The another effective option is radioactive iodine treatment. Iodine is essential for producing thyroide hormones. Radioactive iodine is uptake rapidly and huge amount to follicular cells. I-131 isotope is used for this treatment. It spreads beta particles to surround of uptaked cells and destruct them. If enough amount cells are destructed hyperthyroidism is over. But sometimes much more cells have been destructed. Rest thyroid parenchyma can not produce demanded hormones. So that these patient have to take thyroid hormones externally. However this is preferable situation than hiperthyroidism for doctors. Because hypothyroidism can control easily but hyperthytoidism is both toxic and hardly conrolled disease.
Thyroid gland has important role for metabolism of body because of all reasons. Imbalance of thyroid hormones effects almost all organs and cells. Some genetic mechanisms role on thyroid malfunctions. However sufficient iodine uptake is important to prevent hypothyroidism. Consume iodised salt has encouraged in some countries. Smoking is prohibited for hyperthyroidism patients beacause of aggravated the disease.
References:
- http://www.pathwaymedicine.org/thyroid-hormones
- http://www.pathwaymedicine.org/thyroid-pathology
- https://www.ncbi.nlm.nih.gov/books/NBK285550/
- https://www.endocrineweb.com
- Nuclear Medicine: The Requisites 4th Edition Harvey Ziessman Janis O'Malley
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