Endocrinology: Thyroid

Question 1

Thyroid gland structure:

• ____ gland, ____ gland, highly vascular
• composed of ______, which are filled _____
• also contain ____ cells that release calcitonin.

Answer 1

• single, bi-lobed
• spherical follicles, colloid (glycoprotein)
• parafollicular (C) cells

Question 2

unique aspect of the thyroid

Answer 2

stores a ton of thyroid hormone extracellularly

Question 3

first step in the synthesis of T3/T4

Answer 3

tyrosine-containing Tg produced within the thyroid follicular cells by the ER-golgi complex is transported by exocytosis into the colloid

Question 4

last step of T3/T4 synthesis (at time of secretion)

Answer 4

• on appropriate stimulation (TSH from AP) thyroid follicular cells engulf a portion of Tg-containing colloid by phagocytosis
• lysosomes then attack the engulfed vesicle and split the iodinated products from Tg
• T3 and T4 then diffuse into the blood (secretion).

Question 5

iodine is oxidized to active form by ____ at the luminal membrane

Answer 5

thyroperoxidase - TPO

Question 6

iodide is mounted to tyrosine residues on the Tg molecule, catalyzed by _____, forming _____ or ____

Answer 6

TPO

MIT or DIT

Question 7

Secreted TH = ____% T4, ____%T3

Answer 7

90, 10

Question 8

Plasma T3/T4 bind to _____

Answer 8

thyroxine-binding globulin (TBG); and also albumin but less strongly

Question 9

most ___ converted to ____ in liver and kidneys. this is the main source of ____

Answer 9

T4 T3

T3

Question 10

___ is 10x more potent than ____

Answer 10

T3 T4

Question 11

thyroid hormones have wide spread effects because of ____ actions (mobile receptor).

Answer 11

genomic actions, bc it is lipophilic and binds to receptor in nucleus, creating synthesis of new proteins

Question 12

thyroid hormones have effects on _____ and _____

Answer 12

• BMR/heat production (O2 consumption)

- intermediary metabolism (formation vs degradation of CHO, fat, protein) (hyperthyroid - degradation, hypo - formation)

Question 13

thyroid hormones of ___ effect, especially in heart

Answer 13

sympathy-mimetic, increased B1 receptor expression in heart

Question 14

thyroid hormones have crucial effect on ___ (GH, IGF-1) and _____ development

Answer 14

growth

CNS

Question 15

negative feedback between ____ and ____ accomplishes day to day regulation of free thyroid hormone levels. ____ mediates long-term adjustments

Answer 15

thyroid and AP

hypothalamus

Question 16

Why is the hypothalamo-hypophysiol-thyroid axis so slow?

Answer 16

1) unlike most other hormonal systems, hormones in this axis in an adult do NOT undergo sudden, wide swings in secretion
2) AP (TSH) involved in day to day, Hypothalamus (TRH) involved more long-range
3) TSH IS LIPOPHILIC, THUS ACTS MORE SLOWLY THAN HYDROPHILIC HORMONES
4) LESS THAN 0.1% OF T4 AND LESS THAN 1% OF T3 REMAIN IN THE UNBOUND (FREE) FORM

Question 17

Which thyroid dysfunctions would lead to development of a goiter?

Answer 17

1) primary hypothyroidism - primary failure of the thyroid gland (low T3/T4, high TSH)
2) Hypothyroidisms caused by lack of dietary iodine (Low T3/T4, high TSH)
3) Hyperthyroidism - Graves’ disease (high T3/T4, low TSH)
4) Secondary Hyperthyroidism (High T3/T4, high TRH and/or TSH)

Question 18

Major characteristics of hypothyroidism.

Answer 18

-reduced BMR/cold intolerance
-weight gain (intermediary metabolism)/easily fatigued
-slow heart rate, reflexes, mental responses
-myxedema (complex CHO in skin) - puffy face
-Cretinism - if untreated at birth
TREATMENT: thyroid hormone/dietary iodine

Question 19

major characteristics of hyperthyroidism

Answer 19

• elevated BMR/sweating, heat intolerance
• weight loss/increased appetite, muscle weakness
• rapid heart rate, reflexes, mental responses
• exopthalmos (CHO deposits behind eye) - bulging eyeball
• Graves’ disease
• TREATMENT - anti-thyroid drugs, surgery, radioactive iodine

Question 20

what would anti-thyroid drugs target in treatment of hyperthyroidism?

Answer 20

drugs block the uptake of Iodine by blocking the symporter uptake of Iodine, or drugs could inhibit TPO

Question 21

Key characteristic of Graves’ disease is _____

Answer 21

exophthalmos

Question 22

Why is T3 more potent than T4?

Answer 22

because the nuclear thyroid hormone receptor has 10 times greater affinity for T3 than for T4. thus, T3 is 10x more potent than T4. A hormones potency depends on how strongly the hormone binds to its target cell receptors.

Question 23

TSH acts by increasing _____ in the thyrotropes.

Answer 23

cAMP