Exam #4: Thyroid Gland Flashcards
Describe the anatomy of the thyroid gland. What innervates the thyroid gland. What arteries supply the thyroid gland?
The thyroid gland is located just anterior to the thyroid cartilage and is composed to two lobes connected by an isthmus.
- Vasomotor innervation from the SNS
- Superior and inferior thyroid arteries
Where and how thyroid hormones are made and secreted?
Thyroid hormone is synthesized within the colloid of thyroid follicles, the functional unit of the thyroid gland. These are the steps involved in the formation of thyroid hormones:
1) Continual synthesis and secretion of thyroglobulin into the follicular lumen
2) Iodide trapping, or concentrating iodide in the follicular epithelial cell, via 2Na+-I- symporter (NIS)
3) Movement of iodide into the follicular lumen via another transporter, pendrin
4) Oxidation of iodide to iodine (I- –>I2)
5) Organificiation, or binding iodine to thyroglobulin, catalyzed by thyroid peroxidase
6) Coupling reactions between MIT & DIT
7) Endocytosis of thryoglobulin via megalin
8) Hydrolysis of T3 & T4 from thyroglobulin by lysosomal enzymes & secretion into blood
Describe the functions of the thyroid hormones.
- Generally, thyroid hormone is important for energy homeostasis & regulating energy expenditure; thyroid hormone STIMULATES cell metabolism & activity
- Thyroid hormone is also very important for development, and thyroid deficiencies produce a variety of defects, including severe mental retardation
What are the target cells of the thyroid hormones?
Thyroid hormone receptors are found in virtually all tissues
Where are thyroid hormone receptors located (on cell surface or intracellular)?
The thyroid hormone receptor is located intracellularly; it is bound to or near the Thyroid Response Element (TRE) in DNA
- Thyroid receptor is associated with the Retinoid X Receptor
- In the absence of thyroid hormone, the TRE is repressed
- Thyroid hormone binding causes gene transcription
Describe the effects of too much or too little thyroid hormone.
Thyroid hormone is needed for normal body growth during fetal life, infancy, and childhood
- Too little will result in stunted growth and mental retardation
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*Because of these long-term implications, thyroid levels should be measured at birth
What is cretinism?
This is a form of severe mental retardation that is caused by extreme hypothyroidism in early life
Explain why clinicians measure TSH.
TSH is used as an indirect measure of thyroid hormone levels
- It is assumed that if thyroid hormone levels are LOW, TSH levels are HIGH
- The converse is also assumed, if thyroid hormone levels are HIGH, then TSH is LOW
Describe the negative feedback circuits that control TSH release.
- Thyroid hormone secretion is under the control of the hypothalamus & pituitary gland
- T3 & T4 inhibit TRH release from the hypothalamus & TSH from the anterior pituitary gland
Describe the cause and effects of Graves’ disease.
Grave’s disease is the most common cause of hyperthyroidism and is an autoimmune disorder where autoantibodies stimulate TSH receptors on the thyroid gland. Symptoms include:
- Weight loss accompanied by increased food intake
- Excessive heat production & sweating
- Hair loss
- Exophthalmos
- Goiter
- Tachycardia
- Tremor
- Nervousness
- Weakness
Describe the cause and effects of Hashimoto’s Disease.
Hashimoto’s thyroiditis is a common cause of hypothyroidsm, which is the result of autoantibodies that are directed against thyrolobulin or thyroid peroxidase i.e. destroy the thyroid gland or block thyroid hormone synthesis. Symptoms include:
- Weight gain without increased food intake
- Decreased heat production & cold intolerance
- Bradycardia
- Slowed mental activity
- Periorbital puffiness
- Myxedema leading to thickened/ coarse features
Describe how thyroid hormones are transported in the blood.
Thyroid hormone is transported in the blood mostly bound to plasma proteins
- Major binding protein is thyroxine-binding globulin (TBG)
What is the major thyroid hormone binding protein in the blood?
Thyroxine-binding-globulin (TBG) is the major binding protein in the blood
Is there another specific thyroid binding hormone? If there is what is its name?
Transthyretin (TTR) is another “semi-specific” thyroid-binding protein
- Can bind Thyroxine & Retinol
What other non-thyroid specific proteins bind and transport thyroid hormones?
Albumin
Lipoproteins
