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On this page
  • Parathyroid Embryology
  • Pharyngeal pouch 3
  • Pharyngeal pouch 4
  • Postbranchial body
  • Ectopic thymic and parathyroid tissues
  • Thyroid Formation
  • Thyroglossal duct
  • Ectopic thyroid tissues:
  • Thyroid and parathyroid anatomy
  • isthmus
  • Pyramidal lobe
  • Blood supply
  • Vein drainage
  • Colloid
  • Parafollicular cells
  • Follicular vs parafollicular
  • Parathyroid
  • Thyroid physiology
  • Thyroid hormones
  • T4 vs. T3
  • 5-deiodinase
  • Thyroglobulin
  • Thyroid Hormone Synthesis
  • Thyroid peroxidase
  • Thyroid peroxidase
  • Wolff-Chaikoff effect.
  • Amiodarone
  • Radioactive Iodine
  • Thyroxine-Binding Globulin, aka TBG
  • Thyroid Hormone Receptors
  • Thyroid Hormone functions
  • Thyroid Hormones Metabolic Effect
  • Thyroid Hormones and Bone
  • Thyroid Hormones and CNS
  • Thyroid Hormones and Cardiac Effect
  • Pregnancy and thyroid hormones
  • TRH
  • Thyroid-stimulating immunoglobulins, TSI
  • Calcitonin

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  1. 01 Step 1
  2. Endocrinology

01 Thyroid

PreviousEndocrinologyNext02 Thyroid Disorders

Last updated 5 years ago

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Parathyroid Embryology

Pharyngeal pouch 3

Forms the inferior parathyroid glands (dorsal wing) and thymus (ventral wing).,

Pharyngeal pouch 4

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..

Postbranchial body

Ectopic thymic and parathyroid tissues

Thyroid Formation

Thyroglossal duct

Typically regresses, but may form the adult pyramidal lobe of the thyroid.

A thyroglossal duct cyst/sinus occurs when part of the thyroglossal duct fails to obliterate correctly, subsequently forming a cyst or sinus.

In front of hyoid bone (right):

Moves because still connection with mouth/tongue..

Ectopic thyroid tissues:

  • Puberty/pregnancy: more demand for thyroid hormone, leads to stimulation and growth of thyroid tissue

  • Hypothyroidism stimulates more production of ectopic tissue.,

Thyroid and parathyroid anatomy

isthmus

Pyramidal lobe

Blood supply

Vein drainage

  • Superior thyroid vein — Drains to internal jugular vein

  • Middle thyroid vein — Drains to internal jugular vein

  • Inferior thyroid vein — Drains to the brachiocephalic vein..

  • Follicles

  • Colloid

  • Parafollicular cells..

Thyroid follicles are formed by single layer of epithelial cells known as follicular cells which synthesize, store and secrete thyroid hormone.

Colloid

Parafollicular cells

Follicular vs parafollicular

Parathyroid

Thyroid physiology

Thyroid hormones

  • Thyroxine (T4)

  • Triiodothyronine (T3)

  • Reverse triiodothyronine (rT3)..

T4 vs. T3

  • T4 makes up the majority of thyroid hormone produced. It has a longer half life than T3.

  • T3 is less abundant but more potent than T4.

  • Reverse T3 (rT3) is found in the blood and has no biologic activity..

5-deiodinase

[_](5'-deiodinase inhibited by)Inhibited by

  • propylthiouracil

  • propanolol

  • amiodarone.,

Thyroglobulin

  • large, tyrosine-rich glycoprotein

  • Synthesizes T3/T4

  • resides in the colloid.

  • Made by follicular cells.,

Thyroid Hormone Synthesis

  1. Iodide transport into follicular cells

  2. Iodide oxidized to I2

  3. Tyrosine added to I2, becoming DIT/MIT

  4. DIT/MIT coupling

  5. Proteolysis releases thyroid hormones..

Thyroid peroxidase

  • Found on the luminal surface of the follicular cell, oxidizes I- to I2 (iodine).

  • Conjugates I2 to tyrosine residues on thyroglobulin to form monoiodotyrosine (MIT) and diiodotyrosine (DIT) residues. This reaction is called organification.

    MIT: 1 iodine

    DIT: 2 iodine

  • Couples MIT and DIT.,

[_](What's T4 and T3 coupled from)

  • DIT + DIT = T4

  • MIT + DIT = T3..

  • Thiocyanate [SCN]−

  • Perchlorate

  • Pertechnetate.,

These anions inhibit the uptake of I- into follicular cells, thereby interfering with the synthesis of thyroid hormone.

Thyroid peroxidase

Wolff-Chaikoff effect.

Amiodarone

Radioactive Iodine

Thyroxine-Binding Globulin, aka TBG

  • Thyroxine-binding globulin (TBG) has the highest affinity for T3 and T4, and binds the majority of T3 and T4 in peripheral blood. However, it has the lowest plasma concentration of the three proteins.

  • Albumin carries T3 and T4, and has the highest plasma concentration.

  • Transthyretin (aka pre-albumin) transports T4 only..

TBG acts as pool of T4 to be released..

Thyroid Hormone Receptors

Thyroid Hormone functions

  • Metabolism and growth

  • Glucose, lipid metabolism

  • Cardiac function

  • Bone growth

  • CNS and ANS activity

  • Temperature regulation.,

Thyroid Hormones Metabolic Effect

  • Increasing glycogenolysis and gluconeogenesis

  • Increasing lipolysis

  • increases LDL receptor in liver > decrease serum LDL

  • increase cholesterol secretion in bile

  • decrease cholesterol/TG concentration

  • increases basal metabolic rate (amount of energy used if slept all day)

  • Increasing and upregulating Na/K ATPase activity (more ATP needed, higher RR/temp)..

  • hyperthyroid patient: hyperglycemia, weight loss, tachycardia, tachypnea

  • hypothyroid patient: high cholesterol..

Thyroid Hormones and Bone

Thyroid Hormones and CNS

A hypothyroid neonate is at an increased risk for mental retardation.

Thyroid Hormones and Cardiac Effect

Hyperthyroid patient: tachycardia

[_](4 B's of thyroid function)To recall major thyroid hormone functions, remember the 4 B's:

  • BMR (increase)

  • Bone growth

  • Brain maturation

  • Beta-adrenergic..

Pregnancy and thyroid hormones

TRH

Thyroid-stimulating immunoglobulins, TSI

Calcitonin

  • Increasing osteoblastic activity in bone

  • Decreasing osteoclastic activity in bone

  • Inhibiting intestinal absorption

  • Inhibiting renal tubular reabsorption (promoting excretion in urine)

Minor role in human calcium handling, but used in pharmacologic therapy for hypercalcemia.,

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.

Foramen cecum indicates the former site of the duct.,

Is commonly located ventral to the hyoid bone and laryngeal cartilage.,

Symptoms:

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:

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.,

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:

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,

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).,

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.

Affects temperature regulation by increasing thermogenesis by increase more Na/K ATP pump.,

..

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:

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..

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