Module 1 Section 3 (The Thyroid Gland) Flashcards
Describe the synthesis of thyroid hormone and outline the importance of iodine.
Thyroid synthesis occurs in 5 steps.
1) Tyrosine-containg thyroglobulin is produced within the follicular cells by the ER-Golgi complex -> collpoid (by exocytosis)
2) Iodine is taken up by follicular cells through a process called iodine trapping. Iodine goes against the conc gradient by using a Na-cotransporter that move Na down its conc gradient.
3) Iodine -> colloid of the follicular lumen.
4) The enzyme thyroperoxidase converts iodine -> highly reactive state (I0), which attaches to tyrosine residue on a thyroglobulin molecule (process = iodine organification).
- Attachment of one iodine + tyrosine = monoiodotyrosine (MIT). Meanwhile, the attachment of a second iodine = diiodotyrosine (DIT).
5) coupling proces combined MITs + DITs = thyroid hormones.
- 1 MITs + 1 DITs = T3
- 2 DITs= T4
- Both T3 and T4 remain bound to the thyroglobulin molecule after the chem reaction.
The body required 1mg of iodine per week to ensure sufficient levels of thyroid hormone. The thyroid gland takes up enough thyroid hormone to compensate for long periods w/o it.
- Thyroid hormones can be protected from secretion while they’re stored bound to thyroglobulin in the colloid of the thyroid gland.
Describe the effects of thyroid hormones on metabolism and the consequences of hypo- and
hyperthyroidism.
Metabolic rate
- It incr the overal basal metabolic rate (incr O2 consumpton and energy expenditure)
- consequence of incr metabolic rate = incr heat production
Intermediary metabolism
- Thyroid hormone influences the enzymes involved in fuel metabolism.
- Ex: at low conc of thyroid hormone = conversion of glucose -> glycogen and protein synthesis are favoured. However, at high conc, thyroid hormone causes the breakdown of glycogen -> glucose and the degradation of proteins.
Understanding the control of thyroid hormone release, describe the various hormone levels
during hypothyroidism and hyperthyroidism.
Proper stimulation releases thyroid hormones from the thyroid gland.
1) The follicular cells engulf a portion of the thyroglobulin-containing colloid by phagocytosis and create hormone-filled vesicles.
2) Once inside the follicular cell, lysosomes fuse w/ the vesicles and digestive enzymes release all the MIT, DIT, T3 and T4 from the thyroglobulin.
3) B/c T3 and T4 = lipophilic, they immediately cross the plasma membrane to the blood where they bind to plasma proteins, mainly thyroid-binding globulin.
Describe the conditions that can lead to the formation of a goiter.
Can arise from both hypo- and hyperthyroidism.
It’s an enlarged, visible thyroid gland.
It results from any condition incr TSH since TSH stimulation = incr # and size of follicles
Where is the thyroid gland located?
Over the trachea and just below the larynx.
What does the thyroid gland consist of?
2 lobes connected by a thinner section of the gland (isthmus).
What is the function of the thyroid gland?
Produce and secrete thyroid hormones
The thyroid gland produces 2 hormones from the amino acid tyrosine. What are they?
They are both referred to as thyroid hormone since they have the same physiological effects and both contain iodine. However, they have a difference in speed and intensity of action.
Tetraiodothyronine ( T4 or thyroxine)
- contines 4 iodine molecules and represents about 90% of the thyroid hormones secreted
- T4 -> T3 (in target tissue)
Triiodothyronine (T3)
- considered to be more active
- contains 3 iodine molecules and represents the remaining 10%
True ot false: even compared to other hormones, the actions of thyroid hormone are slow.
True
Can take hrs before the effects are observed and the duration of the response can last for days, even after plasma conc of thyroid hormone have returned to normal.
What are some of the major effects of thyroid hormones?
1) Metabolic rate and heat production
- It incr the overal basal metabolic rate (incr O2 consumpton and energy expenditure)
- consequence of incr metabolic rate = incr heat production
2) Intermediary metabolism
- Thyroid hormone influences the enzymes involved in fuel metabolism.
- Ex: at low conc of thyroid hormone = conversion of glucose -> glycogen and protein synthesis are favoured. However, at high conc, thyroid hormone causes the breakdown of glycogen -> glucose and the degradation of proteins.
3) Symoathomimetic
- Thyroid hormone can incr a target cell’s response to catecholsmines by incr the # of catecholsmine receptors.
4) CV system
- Thyroid hormone has a sympathomimetic effect on the heart. It can incr both heart rate and strength of contraction to incr CO.
- Can incr blood flow.
5) Growth
- It’s essential for normal growth since it stimulate the release of both growth hormone and insulin-like growth factor. It also promotes their actions to stimulate the synthesis of new structural proteins and skeletal growth.
The hypothalamus secretes thyrotropin releasing hormone (TRH) that acts on the anterior pituitary. What does it secrete and why is it important?
It secretes thyroid stimulating hormones (TSH).
1) It’s the most important regulator of thyroid hormone secretion.
2) It also influences most of the stages of thyroid hormone synthesis and release.
3) It directly affects the thyroid gland since if TSH is absent, the thyroid gland shrinks in size. Meanwhile, w/ excess TSH, the gland follicles get larger and incr in #
The release of TSH and TRH are under negative feedback control. Outline the steps.
Stress —(-)—> Hypothalamus ——> TRH —(+)—> Anterior pituitary ——> TSH —(+)—> thyroid gland ——> T3 and T4 ——> incr metabolic rate and heat production; incr of growth and CNS development; incr of sympathetic activity
Negative feedback b/w T3/T4 and anterior pituitary and hypothalamus
What is hypothyroidism?
Low or underactive thyroid. It occurs when the thyroid gland doesn’t secrete enough hormone into the blood.
What are the 3 main causes of hypothyroidism?
Primary failure of the thyroid gland
- Ex: Hashimoto’s thyroiditis (autoimmune disroder where antibodies target thr gland and impair its ability to produce thyroid hormones) -> visibly enlarged thyroid gland
- Low levels of T3 and T4, but high levels of TSH b/w there’s no T3/T4 being produced to provide negative feedback and decrease the production of TRH and TSH
Secondary failure of the thyroid gland
- Happens when the hypothalamus and/or pituitary don’t secrete enough TRH and/or TSH
- Low levels of T3 and/or T4 as well as TRH and/or TSH (depends on location of dysfunction)
Inadequate dietary supply of iodine
- Most common cause
- Low T3 and T4, and elevated TSH
- Ex of foood high in iodine = strawberries
What are the symptoms of hypothyroidism? (6)
- Cold intolerance
- Slower reflexes
- Reduced mental alertness
- Easy to fatigue
- Slow, weak heart rate
- Weight gain b/c of decr basal metabolic rate
