Thyroid Physiology

Question 1

what is the surface anatomy of the thyroid

Answer 1

grows inferiorly around the hyoid and located on the anterior superior portion of the trachea near the third tracheal ring

Question 2

functional unit of the thyroid gland is, describe, function

Answer 2

thyroid follicle

cuboidal epithelial cells (follicular cells) surrounding colloid filled lumen, active follicles smaller

thyroid hormone synthesis

Question 3

what is the function of parafollicular cells, responsive to what

Answer 3

secretion of calcitonin

serum ionized calcium levels

Question 4

describe thyroid hormone synthesis (7 steps)

Answer 4

1. iodide uptake by Na/I transporter
2. thyroglobulin synthesis by RER - secreted into follicle lumen
3. Tg iodination mediated by TPO
4. I-Tg stored in a colloidal form
5. Colloidal Tg delivered to lysosomes
6. Lysosomes break down Tg to individual amino acids, T3, T4, MIT and DIT
7. T3 and T4 diffuse back into the blood steam

Question 5

which is the predominant TH released

Answer 5

T4, 10x as much

Question 6

what stimulates TH synthesis

Answer 6

thyroid stimulating hormone (TSH), binds follicular cell receptor, active transport of iodide and synthesis of Tg

Question 7

2 functions of TPO

Answer 7

conversion of I- –> I

coupling of iodide to tyrosine and Tg

Question 8

what causes the release of TSH, what tissues are involved

Answer 8

hypothalamus secretes thyrotropin releasing hormone (TRH), causes release of TSH from the anterior pituitary

Question 9

describe the T4 feedback loop

Answer 9

T4 increase causes decreased sensitivity of the A.pit to TRH, also causes reduced secretion o TRH by hypothalamus

Question 10

describe activity and half life of T3 and T4

Answer 10

T3 more active, shorted half life

T4 less active, longer half life

Question 11

describe the embryology of the thyroid

Answer 11

invagination of endoderm of the first branchial pouch near lingual bud.

Question 12

How are thyroid hormones transported

Answer 12

Bound to serum proteins (99%)
Mainly thyroxine binding globulin (70-80%), also thyroxine binding prealbumin (t4 - ~10%)
Serum albumin also when higher affinity proteins saturated

Question 13

Half life of T3 and T4

Answer 13

T4 - 5-7 days, T3 - 1 day

Question 14

Symptoms of hyperthyroidism

Answer 14

Decrease weight, increased basal metabolic rate, heat tolerance, increased heart rate, irritable, most skin, exophthalmos, goitre

Question 15

Symptoms of hypothyroidism

Answer 15

Increased weight, decreased basal metabolic rate, cold intolerance, decreased heart rate, sluggish, dry skin, goitre, decreased mental development, decreased growth

Question 16

Causes of hyperthyroidism

Answer 16

Graves (causes 60-80%), MNG, toxic solitary adenoma , pituitary tumour, thyroiditis, iodine induced, thyroid carcinoma

Question 17

Symptoms of thyroid eye disease

Answer 17

Grittiness, lid retraction, proptosis, optic neuropathy, ophthalmoplegia