38: Thyroid Gland

Question 1

What cell type secretes thyroid hormones

Answer 1

Follicular epithelium

Question 2

Outermost part of thyroid follicle

Answer 2

Basal lamina

Question 3

Follicular lumen contents

Answer 3

Contains colloid, which contains newly synthesized thyroid hormones attached to thyroglobulin, which stores iodine as tyrosine

Question 4

How much hormone is stored in the thyroid + how much is secreted daily

Answer 4

Stored: enough for 2-3 months
Ssecrete: 60 ug daily

Question 5

Parafollicular / C cell function

Answer 5

Secrete calcitonin

Question 6

Name of T3 and T4

Answer 6

T3: triiodothyronine
T4: tetraiodothyronine

Question 7

Which is the active form, T3 or T4?

Answer 7

T3

Question 8

Which is secreted more, T3 or T4

Answer 8

T4 (10x more)

Question 9

Half life of T3 and T4

Answer 9

T3: 1 day
T4: 6 days

Question 10

How much T3 comes directly from thyroid vs peripheral conversion

Answer 10

Thyroid: 1–20%
Conversion: 80-90%

Question 11

Enzyme that converts T4 to T3 in circulation

Answer 11

Deiodinase

Question 12

Three things that reduce peripheral conversion to T3

Answer 12

1. Fasting
2. Medical stress
3. Catabolic disease

Question 13

Pendrin

Answer 13

A Cl/I pump involved in thyroid hormone synthesis, in the apical membrane of follicular cells

Question 14

Mutation in Pendrin gene / Pendren syndrome

Answer 14

Defect in transport of iodine across apical membrane -> hypothyroidism with goiter + sensorineural hearing loss

Question 15

Wolff-Chaikoff effect

Answer 15

High levels of I inhibit organification and synthesis of thyroid hormones

Question 16

What can deficiency of deiodinase mimic?

Answer 16

Dietary deficiency in Iodine

Question 17

What thyroid hormone is favored to be created with a restricted availability of iodine?

Answer 17

T3 is favored

Question 18

Three main thyroid hormone binding proteins in the blood + their percentages

Answer 18

1. TBG: thyroxine-binding globulin: 70%
2. TRR: transthyretin: 10-15%
3. Albumin: 15-20%

Question 19

Where is TBG synthesized?

Answer 19

Liver

Question 20

T3 Resin Uptake Test function

Answer 20

Measures circulating levels of TBG to measure levels of bound hormone

Question 21

How T3 resin uptake test works

Answer 21

1. Start with TBG bound to T4 from serum
2. Add unbound and labeled T3, which will bind unbound TBG sites
3. Anti-T3 Ab resin adsorbs T3, separating out T3 from T4 - can be measured at this point

Question 22

Level of T4 and T3 in high and low TBG states

Answer 22

High TBG: high T4, low T3

Low TBG: low T4, high T3

Question 23

Levels of TBG and T3 in hepatic failure**

Answer 23

Low TBG, high T3

Question 24

Hepatic failure effect on thyroid hormone

Answer 24

It causes a transient increase in free T3 and T4, followed by an inhibition of synthesis of T3 and T4

Question 25

TBG and T3 in pregnancy

Answer 25

High TBG, low T3

Question 26

Conditions that cause high and low TBG

Answer 26

High TBG: pregnancy | Low TBG: hepatic failure

Question 27

Pregnancy and thyroid hormones

Answer 27

Decrease in free T3 and T4 -> increased synthesis of T3 and T4 - >increased total levels of T3 and T4, but since also high TBG, levels of free H levels are normal -> pt is euthyroid

Question 28

Thyroid hormones cause synthesis of what proteins in most tissues?

Answer 28

Na/K ATPase, transport proteins, lysosomal and proteolytic enzymes, structural proteins

Question 29

Three proteins up regulated by thyroid hormones in cardiac muscle

Answer 29

Myosin, B1-adrenergics, Ca ATPase

Question 30

Five causes of goiter

Answer 30

1. Grave’s disease 2. Secondary hyperthyroidism 3. Lack of iodine 4. Sporadic hypothyroid 5. Hashimoto’s

Question 31

two things TSH does to the thyroid

Answer 31

Regulates secretion of thyroid hormones + BUT also regulates growth of the gland

Question 32

Secretion of Thyroid hormone vs GH

Answer 32

Thyroid: secreted in a steady state GH: secretion in waves

Question 33

Plasma cholesterol and triglycerides vs thyroid hormone

Answer 33

Inversely related

Question 34

Neural changes that cause congenital hypothyroidism

Answer 34

Abnormal development of synapses + decreased dendritic branching and myelination -> neural changes irreversible unless replacement therapy starts soon after birth