SM154 Thyroid Basic

Question 1

Basic functional unit of thyroid

Answer 1

Follicle, a hollow sphere filled with thyroglobulin surrounded by thyroid follicular cells and a basal membrane

Question 2

C cells

Answer 2

Adjacent to follicles, secrete calcitonin

Question 3

Basic anatomy of thyroid gland

Answer 3

Left and right lobes connected by an isthmus, inferior to cricoid cartilage on anterior esophagus, close to recurrent laryngeal nerves

Question 4

Describe the hypothalamic-pituitary-thyroid axis

Answer 4

TRH (hypothalamus) stimulates the secretion of TSH (pituitary), which stimulates the thalamus to secrete more T4 and T3. T3 feedbacks to inhibit release of both TSH and TRH. SS and dopamine inhibit release of TSH.

Question 5

How does iodide enter a thyroid follicular cell?

Answer 5

Via the sodium-iodide symporter (NIS) on the basolateral membrane

Question 6

What does thyroperoxidase do? Where is it located?

Answer 6

In thyroid follicular cells, 1) oxidizes iodide, 2) organifies iodide, 3) couples MIT and DIT

Question 7

What is produced when TPO organifies iodide?

Answer 7

MIT and DIT

Question 8

What does thyroxine-binding globulin do? When might it be elevated or low?

Answer 8

TBG binds free T3 and T4 in the periphery and keeps it inactive. Elevated: pregnancy and birth control and low in hepatic failure

Question 9

What converts T4 to T3? How do you get rT3? Which is active?

Answer 9

5’-deiodinase gives you T3
5-deiodinase gives you rT3
T3 is active, rT3 is inactive

Question 10

3 major thyroid hormone binding proteins

Answer 10

Thyroxine-binding globulin (TBG) > transthyretin (TTR) > albumin

Question 11

Two types of thyroid hormone receptors

Answer 11

TRa and TRb

Question 12

TSH measurement vs. T4/T3 assays

Answer 12

TSH is the best biochemical marker of thyroid function, T4/T3 are less reliable

Question 13

Relationship between FT4 and TSH

Answer 13

Inverse log-linear relationship: small change in FT4 produces a big change in TSH

Question 14

Thyroglobulin test: when might TG be detectable?

Answer 14

Residual or recurrent thyroid tissue (the more thyroid tissue, the higher the TG)

Examples include goiter, hyperthyroidism, thyroiditis (acutely), thyroid tumors

Question 15

Thyroglobulin antibodies: prevalence in normal population, prevalence in cancer, effects on TG measurement

Answer 15

10%, 20%, interferes with it

Question 16

Modality of choice for thyroid imaging

Answer 16

Ultrasound

Question 17

What is thyroid ultrasound good for?

Answer 17

Morphological analysis and size analysis of nodules, and for fine needle aspiration (FNA) guiding

Question 18

Ultrasound characteristics of a cyst

Answer 18

Echo-free center (completely black), smooth back wall, acoustic enhancement

Question 19

Ultrasound characteristics of a solid nodule

Answer 19

Center is non-echo free (not black), back wall is indistinct, no acoustic enhancement

Question 20

Ultrasound characteristics of a complex nodule

Answer 20

Characteristics of both solid nodules and cysts

Question 21

What isotopes are used in scans? What is the name of the test you use? What does a high value suggest vs. a low value?

Answer 21

123-I, 131-I, 99-technetium
RAIU: radioiodine uptake
High value suggests hyperthyroidism, low value suggests hypothyroidism

Question 22

What is 131-I used for at low dose vs. high dose? What does it emit?

Answer 22

Low dose: Grave’s disease, nodular goiter
High dose: thyroid cancer (destruction)
Emits beta particles