Module 1 Section 3 (The Thyroid Gland)

Question 1

Describe the synthesis of thyroid hormone and outline the importance of iodine.

Answer 1

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.

Question 2

Describe the effects of thyroid hormones on metabolism and the consequences of hypo- and
hyperthyroidism.

Answer 2

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.

Question 3

Understanding the control of thyroid hormone release, describe the various hormone levels
during hypothyroidism and hyperthyroidism.

Answer 3

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.

Question 4

Describe the conditions that can lead to the formation of a goiter.

Answer 4

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

Question 5

Where is the thyroid gland located?

Answer 5

Over the trachea and just below the larynx.

Question 6

What does the thyroid gland consist of?

Answer 6

2 lobes connected by a thinner section of the gland (isthmus).

Question 7

What is the function of the thyroid gland?

Answer 7

Produce and secrete thyroid hormones

Question 8

The thyroid gland produces 2 hormones from the amino acid tyrosine. What are they?

Answer 8

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%

Question 9

True ot false: even compared to other hormones, the actions of thyroid hormone are slow.

Answer 9

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.

Question 10

What are some of the major effects of thyroid hormones?

Answer 10

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.

Question 11

The hypothalamus secretes thyrotropin releasing hormone (TRH) that acts on the anterior pituitary. What does it secrete and why is it important?

Answer 11

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 #

Question 12

The release of TSH and TRH are under negative feedback control. Outline the steps.

Answer 12

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

Question 13

What is hypothyroidism?

Answer 13

Low or underactive thyroid. It occurs when the thyroid gland doesn’t secrete enough hormone into the blood.

Question 14

What are the 3 main causes of hypothyroidism?

Answer 14

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

Question 15

What are the symptoms of hypothyroidism? (6)

Answer 15

• Cold intolerance
• Slower reflexes
• Reduced mental alertness
• Easy to fatigue
• Slow, weak heart rate
• Weight gain b/c of decr basal metabolic rate

Question 16

What is congenital hypothyroidism (cretinism)?

Answer 16

It’s a thyroid hormone deficiency acquired from birth.

It’s characterized by drawfism and intellectual disability.

Question 17

What is hyperthyroidism?

Answer 17

Increased levels of thyroid hormone.

Question 18

What are the 3 main causes of hyperthyroidism?

Answer 18

Secondary to excess hypothalamic or anterior pituitary secretions

• Tumor in the hypothalamus (secretes excess TRH) or in the anterior pituitary (secretes excess TSH).
• Those tumors ignore negative feedback -> high levels of T3 and T4, w/ elevated TRH and/or TSH.

Thyroid tumour

• Tumor in the thyroid gland -> incr sectrion of thyroid hormones
• High levels of T3 and T4 and decr TSH

Graves’ disease

• Most common
• Autoimmune disease where body produces long-acting thyroid stimulator (LATS), an antibody that targets/activates TSH receptors on follicular cells
• LATS has the same effect as excess TSH -> follicles then grow larger and incr in #
• Not under negative feedback control, so levels of T3 and T4 are high while TSH is low

Question 19

What are the symptoms of hyperthyroidism? (5)

Answer 19

• Incr heart rate
• Excessive heat production
• Muscle weakness b/c of skeletal muscle protein degradation
• Mood swings b/c of incr CNS mental alertness
• Elevated basal metabolic rate -> weight loss even w/ incr caloric intake

Question 20

Predict whether a goiter will be present in each of the 3 main causes of hypothyroidism.

• Primary failure of thyroid =
• Secondary failure of the thyroid gland =
• Lack of dietary iodine =

Answer 20

• Primary failure of thyroid = Goiter
• Secondary failure of the thyroid gland = No goiter
• Lack of dietary iodine = Goiter

Question 21

Predict whether a goiter will be present in each of the 3 main causes of hyperthyroidism.

• Secondary to hypothalamus or anterior pituitary tumour =
• Hypersecreting thyroid tumour =
• Graves disease =

Answer 21

• Secondary to hypothalamus or anterior pituitary tumour = Goiter
• Hypersecreting thyroid tumour = No goiter
• Graves disease = Goiter