Thyroid Anatomy and Physiology

Question 1

What does the thyroid gland consist of

Answer 1

Left and right lobes connected by a midline isthmus

Question 2

Where does the isthmus lie

Answer 2

Below the cricoid cartilage

Question 3

What covers the thyroid gland

Answer 3

Strap muscles of the neck and overlapped by the sternocleidomastoids

Question 4

What encloses the thyroid gland and attaches it to the larynx and the trachea ?

Answer 4

The pre tracheal fascia

Question 5

What accounts for the upward movement of the thyroid gland on swallowing

Answer 5

The pre tracheal fascia attaching it to the larynx

Question 6

Where does the thyroid gland develop from

Answer 6

The floor of the pharynx in the position of the foramen caecum of the adult tongue

Question 7

Down which structure does the thyroid gland move to during development

Answer 7

Thyroglossal duct

Question 8

What is the thyroid gland composed of

Answer 8

Epithelial spheres called follicles, whose lumens are filled with a proteinaceous colloid containing thyroglobulin

Question 9

What are the two basic cell types present in the follicles of the thyroid and what do they each secrete

Answer 9

Follicular cells - secrete thyroxine (T4) and triiodothyronine (T3)
Parafollicular or C cells - secrete calcitonin

Question 10

What sort of things do thyroid hormones regular

Answer 10

Organogenesis, growth and development 
Energy expenditure 
protein, carbohydrate and fat metabolism
gut motility 
bone turnover
heart rate and contractility and peripheral vascular resistance 
beta-adrenergic receptor expression
muscle contraction and relaxation
menstrual cycle 
erythropoiesis (RBC production)

Question 11

What is essential for normal thyroid function and how do we obtain it

Answer 11

Iodine

Obtained by ingestion of food such as seafood, seaweed, kelp, dairy, some votable and iodised salt

Question 12

What is the recommended iodine intake for adults

Answer 12

150ug / day

Question 13

Where is thyroglobulin synthesised

Answer 13

In the RER and is transported into the follicular lumen by exocytosis

Question 14

How is iodine transported into the thyroid follicular cells

Answer 14

Via a sodium-iodide symporter on the basolateral membrane of the follicular cells

Question 15

What is the enzyme that catalyses the process of oxidation of iodide to iodine and its binding (organification) to the tyrosine residues of thyroglobulin to form MIT and DIT

Answer 15

Thyroid peroxidase enzyme

Question 16

What are DIT and MIT molecules linked by

Answer 16

TPO to form thyroxine and triiodothyronine in a process known as coupling

Question 17

What are uncoupled MIT and DIT

Answer 17

Deoiodinated, the free tyrosine and iodide are recycled

Question 18

What does the thyroid gland store

Answer 18

T4 and T3 incorporated in thyroglobulin and can therefore secrete T4 and T3 more quickly than if they had to be synthesised

Question 19

What structure produces T4

Answer 19

Entirely the thyroid gland

Question 20

What might changes in T3 concentration indicate

Answer 20

A change in th rate of peripheral conversion

May not be an accurate measure of the change in tyroid hormone production

Question 21

What are the majority of circulating T4 and T3 bound to

Answer 21

Plasma proteins

Question 22

Only the bound thyroid hormone is available to the tissues. True or False

Answer 22

False - only the unbound form is available to the tissues

Question 23

What are some things that could cause an increase in thyroid binding globulin levels

Answer 23

Hereditary TBG excess (X-linked) 
pregnancy 
drugs - oestrogen, tamoxifen, opiates,
Hepatitis 
Acute intermittent porphyria

Question 24

What are some things that could cause a decrease in thyroid binding globulin levels

Answer 24

Genetically determined 
malnutrition
chronic liver disease
nephrotic syndrome 
drugs - androgens, corticosteroids, 
Cushing's syndrome 
Acromegaly

Question 25

What stimulates the synthesis and secretion of T3 and T4

Answer 25

Thyroid-stimualting hormone (TSH)

Question 26

Where is TSH released from

Answer 26

Anterior pituitary gland

Question 27

What increases the release and production of TSH

Answer 27

hypothalamic thyrotrophin-releasing hormone

Question 28

What is TSH

Answer 28

a glycoprotein secreted by the thyrotroph cells of the anterior pituitary Composed of alpha and beta subunits that are non-covalently bound

Question 29

Alpha subunit of TSH is unique to TSH. true or false

Answer 29

False - it is the beta that is unique to TSH | Alpha is the same as that of LH, FSH and human chorionic gonadotrophin

Question 30

What are some causes of increased TSH secretion

Answer 30

``` Primary hypothyroidism Secondary hyperthyroidism Thyroid hormone resistance Primary adrenal insufficiency Dopamine antagonists amiodarone ```

Question 31

What are some causes of decreased TSH secretion

Answer 31

Primary hyperthyroidism Secondary hypothyroidism Dopamine agonists steroids

Question 32

How do thyroid hormones enter cells

Answer 32

Via active membrane transporter proteins

Question 33

What is the majority of T3 produced by

Answer 33

5' deiodination of T4 in peripheral extra-thyroidal tissues (liver and kidney)

Question 34

What is easier to interpret - free or total thyroid hormone and why

Answer 34

Free | The level of bound hormone alters with changes in the levels of thyroid -binding proteins