Forms the inferior parathyroid glands (dorsal wing) and thymus (ventral wing).
Forms the superior parathyroid glands (dorsal wing) and ultimobranchial body (ventral).
Note that the parathyroid glands formed by pharyngeal pouch 3 migrate below the parathyroid glands of pharyngeal pouch 4..
Neural crest cells subsequently migrate into the ultimobranchial body to form parafollicular C-cells of the thyroid gland..
Aka ultimobranchial body.
can be found in the lateral regions of the neck along its path of migration.
On day 24, the primitive pharynx of the embryological foregut derives the thyroid diverticulum which in turn forms the thyroid gland. Foregut, between copula and Tuberculum impar..
The thyroid diverticulum migrates caudally down the midline, ventral to the hyoid bone and laryngeal cartilages, to its adult anatomic position..
The thyroid remains connected to the tongue via this duct during thyroid development and migration, which later obliterates.
Typically regresses, but may form the adult pyramidal lobe of the thyroid.
Foramen cecum indicates the former site of the duct.
A thyroglossal duct cyst/sinus occurs when part of the thyroglossal duct fails to obliterate correctly, subsequently forming a cyst or sinus.
Is commonly located ventral to the hyoid bone and laryngeal cartilage.
Symptoms:
In front of hyoid bone (right):
Moves because still connection with mouth/tongue..
A lingual cyst occurs when a thyroglossal duct cyst is located at the base of the tongue..
The thyroid gland consists of 2 lobes located anterolaterally, which span the area between the oblique line of the thyroid cartilage and the 5th tracheal ring..
Thin band of tissues between 2 lobes.
Lobe above isthumas. Remnant of thyroglossal duct.
The thyroid receives arterial supply from the superior (via external carotid) and inferior thyroid (via thyrocervical trunk of subclavian) arteries..
Three separate veins provide drainage of the thyroid gland:
The functional anatomy of the thyroid gland consists of:
Thyroid follicles are formed by single layer of epithelial cells known as follicular cells which synthesize, store and secrete thyroid hormone.
Found within the lumen of each thyroid follicle. In this compartment, thyroid hormone is stored as a component of thyroglobulin.
Synthesize and secrete the hormone calcitonin.
The follicular cells of the thyroid gland are derived from endoderm. Parafollicular C cells are derived from neural crest cells..
These are 4 small circular glands found on the posterior surface of the thyroid gland. Clinical Correlate: Thyroid surgery is a common cause of removal or damage to the parathyroid glands..
Contain element Iodine, found in table salt..
In order to be incorporated into thyroid hormones, iodine needs to be
..
There are three types of thyroid hormone:
Deiodinate T4 to T3 in peripheral tissues.
Inhibited by
propanolol: for thyrotoxin. Catecholamine can result in hyperthyroidism
Also amiodarone:
Overview steps of thyroid hormone synthesis:
Iodide (I-) is transported into the follicular cell via the Na-I cotransporter and then into the lumen of the follicle..
Iodinated thyroglobulin is stored in colloid within the follicular lumen. Upon stimulation by TSH, follicular cells endocytose thyroglobulin and lysosomal enzymes digest thyroglobulin, secreting T3 and T4 into the circulation..
Residual MIT and DIT are deiodinated by thyroid deiodinase. The I2 that is generated is recycled to synthesize more thyroid hormone..
The Na-I transporter is competitively inhibited by anions including:
These anions inhibit the uptake of I- into follicular cells, thereby interfering with the synthesis of thyroid hormone.
Inhibited by propylthiouracil and methimazole.
High levels of iodine inhibit thyroid hormone synthesis by blocking the organification step. Less synthesis of MIT/DIT.
Competes with T4 Check TSH level in patients before start amiodarone..
..
Three proteins are involved in the transport of T3 and T4 in the blood:
Only free T3 or T4 is active, whereas TBG-bound T3 or T4 is inactive. However,
TBG acts as pool of T4 to be released..
Conditions where TBG is increased (e.g. pregnancy) or decreased (e.g. liver failure) can affect total T3 or T4 levels. Increased estrogen during pregnancy causes increased TBG, which results in increased total T3 and T4 levels. However, the free T3 and free T4 levels are actually normal (therefore, pregnancy is not a hyperthyroid state)..
these are..
It mediates:
Increases basal metabolic rate and oxygen consumption via multiple mechanisms, including:
Affects growth and maturation by promoting bone growth/turnover, and acting synergistically with growth hormone and insulin-like growth factor-1 (IGF-1).
Is important in perinatal maturation of the central nervous system (CNS).
A hypothyroid neonate is at an increased risk for mental retardation.
Affects the autonomic nervous system (ANS) by increasing sympathetic activity (β-adrenergic tone). For example, thyroid hormone leads to the up-regulation of β1-adrenergic receptors in the heart.
Hyperthyroid patient: tachycardia
Affects temperature regulation by increasing thermogenesis by increase more Na/K ATP pump.
To recall major thyroid hormone functions, remember the 4 B's:
..
Thyrotropin releasing hormone (TRH) from the hypothalamus stimulates the secretion of thyroid stimulating hormone (TSH) by the anterior pituitary..
TSH from anterior pituitary stimulates increased synthesis and secretion of thyroid hormones, by binding follicular cell receptors and increasing intracellular levels of cAMP (adenylate cyclase-cAMP mechanism)..
T3 and T4 down-regulate TRH receptors in the anterior pituitary leading to an inhibition of TSH secretion. This is an example of negative feedback.. T3/T4 levels sensed by hypothalamus
Auto-antibodies (IgG) against TSH receptors. Binding to TSH receptors on follicular cells stimulates T3/T4 release. Patients with Graves disease exhibit high concentrations of TSI.
..
Produced by the parafollicular cells (C cells), serves to lower blood calcium levels. It does this by several mechanisms including:
Minor role in human calcium handling, but used in pharmacologic therapy for hypercalcemia.
The stimulus for calcitonin secretion is increased serum calcium concentration..
Calcitonin opposes the actions of parathyroid hormone and is not important in normal Ca2+ homeostasis..