Thyroid Gland is a vital butterfly-shaped endocrine gland situated in the lower part of the neck. It is present in front and sides of the trachea, inferior to the larynx. It plays an essential role in the regulation of the basal metabolic rate (BMR), and stimulates somatic and psychic growth, besides having a vital role in calcium metabolism.
The thyroid gland is a midline structure located in the anterior neck. The thyroid functions as an endocrine gland and is responsible for producing thyroid hormone and calcitonin, thus contributing to the regulation of metabolism, growth, and serum concentrations of electrolytes such as calcium.[1][2]
Many disease processes can involve the thyroid gland, and alterations in the production of hormones can result in hypothyroidism or hyperthyroidism. The thyroid gland is involved in inflammatory processes (e.g., thyroiditis), autoimmune processes (e.g., Graves disease), and cancers (e.g., papillary thyroid carcinoma, medullary thyroid carcinoma, and follicular carcinoma).
Anatomy of Thyroid Gland
The thyroid gland is divided into two lobes that are connected by the isthmus, which crosses the midline of the upper trachea at the second and third tracheal rings. In its anatomic position, the thyroid gland lies posterior to the sternothyroid and sternohyoid muscles, wrapping around the cricoid cartilage and tracheal rings. It is located inferior to the laryngeal thyroid cartilage, typically corresponding to the vertebral levels C5-T1. The thyroid attaches to the trachea via consolidation of connective tissue, referred to as the lateral suspensory ligament or Berry’s ligament. This ligament connects each of the thyroid lobes to the trachea. The thyroid gland, along with the esophagus, pharynx, and trachea, is found within the visceral compartment of the neck which is bound by pretracheal fascia.
It is a gland consisting of two lobes, the right and the left lobes joined together by an intermediate structure, the isthmus. Sometimes a third lobe called the pyramidal lobe projects from the isthmus. It has a fibrous/fibromuscular band, i.e., levator glandular thyroidal running from the body of the hyoid to the isthmus.[rx] The lobes are 5 x 2.5 x 2.5 cm in dimension and weight around 25 gm. It extends from the fifth cervical to the first thoracic vertebrae. The lobes extend from the middle of the thyroid cartilage to the fifth tracheal rings. The isthmus is 1.2 x 1.2 cm in dimensions and extends from second to third tracheal rings. It grows larger in females during the period of menstruation and pregnancy.
The lobes are conical in shape and have an apex, a base, three surfaces – lateral, medial and posterolateral, and two borders – the anterior and posterior. The isthmus, however, has two surfaces – anterior and posterior and two borders – superior and inferior.
The lobes are related anteriorly to the skin, superficial and deep fascia, and platysma. Posteriorly, the lobes are associated with the laminae of the thyroid cartilage and tracheal rings, and laterally to the external carotid artery and internal jugular vein.
The thyroid gland is a richly vascular organ supplied by the superior and inferior thyroid arteries and sometimes by an additional artery known as thyroidea ima artery.[rx] The venous drainage is by superior, middle, and inferior thyroid veins. Sometimes a fourth thyroid vein might be present called the vein of Kocher. The nerve supply is mainly from middle cervical ganglion, but also partly from superior and inferior cervical ganglions.
Physiology of Thyroid Gland
Stressors stimulate increased Thyroid function
- Puberty
- Pregnancy
- Physiologic stress
Hypothalamus
- Thyrotropin Releasing Hormone (TRH) released
- TRH passes down the pituitary stalk to the pituitary
Anterior Pituitary
- Thyroid Stimulating Hormone (TSH) released
- TSH passes into circulation to Thyroid
Thyroid
- Iodine is absorbed from circulation
- Thyroxine (T4) released into circulation
- Triiodothyronine (T3) is also 20-30% produced in the Thyroid (the remainder is the peripheral conversion of T4)
Peripheral Tissues
- Triiodothyronine (T3) produced from T4 deiodination
Circulation
- Thyroid Binding Globulin (TBG) binds T4 and T3
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- T4 is 99.96% protein-bound in serum (<1% is Free T4, the active form)
- T3 is 99.7% protein-bound in serum
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- T4 and T3 stimulate all metabolic processes
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- T3 is several times more potent than T4
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Effects of Thyroid hormone
- Peripheral tissue oxygen utilization and basal metabolic rate increased
- Positive chronotropic and ionotropic cardiovascular effects, with secondary increased Cardiac Output
- Decreases systemic vascular resistance via arteriole dilation
- When Thyroid hormone is in excess (e.g. Thyroid Storm), high output Heart Failure may result
Feedback Loop
- Increased T4 and T3 levels suppress TRH and TSH
Nerves of Thyroid Gland
- The thyroid gland is in very close proximity to two important nerves – the external branch of the superior laryngeal nerve and also the recurrent laryngeal nerve. Both are branches of the vagus nerve. During thyroidectomy, damage to these nerves leads to disability in phonation and/or difficulty in breathing.
- Injury to one of the branches of the superior laryngeal nerve leads to great difficulty in singing. Injury to the recurrent laryngeal nerve unilaterally may lead to hoarseness of the voice, and difficulty in breathing. Bilateral recurrent laryngeal nerve injury is serious and often necessitates the need for a tracheostomy.[rx]
- The autonomic nervous system primarily innervates the thyroid gland. The vagus nerve provides the main parasympathetic fibers, while sympathetic fibers originate from the inferior, middle, and superior ganglia of the sympathetic trunk. These nerves do not play a role in the control of hormonal production or secretion but mostly influence vasculature. [rx]
Muscles of Thyroid Gland
Several muscles should be considered when discussing neck and thyroid surgical anatomy.
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Platysma – The first muscle encountered during neck dissection, it is enveloped by the superficial cervical fascia. It sits in the anterior neck and extends from the superficial fascia of the deltoid, over the clavicle, reaching the mandible and superficial fascia of the face superiorly.
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Sternocleidomastoid – This muscle forms the anterior portion of the posterior triangle of the neck. The muscle runs obliquely from the mastoid to the clavicle and sternum. The sternocleidomastoid is found anterolaterally relative to the thyroid gland.
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Digastric muscle – This muscle extends from the mandibular tubercle, passes deep and inferior to the hyoid, and loops back up to attach to the mastoid tip.
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Infrahyoid muscles – These are also referred to as “strap muscles.” They include four paired muscles found on the anterolateral surface of the thyroid gland. The strap muscles result in gross movement of the larynx during swallowing and also adjust the positioning of the larynx during vocalization.
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Omohyoid muscle – The omohyoid muscle is found deep in the sternocleidomastoid. It extends from the hyoid bone to the lateral aspect of the clavicle.
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Sternohyoid muscle – This muscle sits anterior the remaining strap muscles and the thyroid gland. The sternohyoid muscle extends from its superior attachment at the hyoid bone inferiorly to the sternum.
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Sternothyroid muscle – This muscle extends from the oblique line of the thyroid cartilage to the sternum. This muscle contacts the anterior surface of the thyroid gland.
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Thyrohyoid muscle – The thyrohyoid muscle extends from the oblique line of the thyroid cartilage to the hyoid bone superiorly.
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Inferior pharyngeal constrictor – This muscle extends from its anterior attachment at the oblique line of the thyroid cartilage and lateral aspect of the cricoid cartilage to the pharyngeal raphe. This muscle contacts the superior pole of the lateral lobe of the thyroid gland medially.
Function of Thyroid Gland
Some of the essential functions of the thyroid hormones are as follows:
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They regulate the basal metabolic rate (BMR).
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They play an important role in calcium metabolism
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They help in the overall development and function of CNS in children.[rx]
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They stimulate somatic and psychic growth.
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They stimulate heart rate and contraction.[rx]
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They help in the deposition of calcium and phosphate in bone and make the bones strong.
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They decrease the level of calcium in the blood.
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They regulate carbohydrate, fat, and protein metabolism.
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They also help in the metabolism of vitamins.
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They regulate the body temperature.
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They help degrade cholesterol and triglycerides.
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They maintain the electrolyte balance.
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They support the process of RBC formation.
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They enhance mitochondrial metabolism.
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They increase the oxygen consumptions by the cells and tissues.
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They influence the mood and behavior of a person.[rx]
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They stimulate gut motility.[rx]
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They also enhance the sensitivity of the beta-adrenergic receptors to catecholamines.
Thus the thyroid hormones act on almost all the cells of the body. They also take up a key role in the development, growth, and function of most of the tissues and organs of the body. One can also say that the thyroid hormones are mandatory for the normal metabolic activity of all the cells of the body.
References
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