Endocrinology: Thyroid Flashcards
Thyroid gland structure:
- ____ gland, ____ gland, highly vascular
- composed of ______, which are filled _____
- also contain ____ cells that release calcitonin.
- single, bi-lobed
- spherical follicles, colloid (glycoprotein)
- parafollicular (C) cells
unique aspect of the thyroid
stores a ton of thyroid hormone extracellularly
first step in the synthesis of T3/T4
tyrosine-containing Tg produced within the thyroid follicular cells by the ER-golgi complex is transported by exocytosis into the colloid
last step of T3/T4 synthesis (at time of secretion)
- 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).
iodine is oxidized to active form by ____ at the luminal membrane
thyroperoxidase - TPO
iodide is mounted to tyrosine residues on the Tg molecule, catalyzed by _____, forming _____ or ____
TPO
MIT or DIT
Secreted TH = ____% T4, ____%T3
90, 10
Plasma T3/T4 bind to _____
thyroxine-binding globulin (TBG); and also albumin but less strongly
most ___ converted to ____ in liver and kidneys. this is the main source of ____
T4 T3
T3
___ is 10x more potent than ____
T3 T4
thyroid hormones have wide spread effects because of ____ actions (mobile receptor).
genomic actions, bc it is lipophilic and binds to receptor in nucleus, creating synthesis of new proteins
thyroid hormones have effects on _____ and _____
- BMR/heat production (O2 consumption)
- intermediary metabolism (formation vs degradation of CHO, fat, protein) (hyperthyroid - degradation, hypo - formation)
thyroid hormones of ___ effect, especially in heart
sympathy-mimetic, increased B1 receptor expression in heart
thyroid hormones have crucial effect on ___ (GH, IGF-1) and _____ development
growth
CNS
negative feedback between ____ and ____ accomplishes day to day regulation of free thyroid hormone levels. ____ mediates long-term adjustments
thyroid and AP
hypothalamus
Why is the hypothalamo-hypophysiol-thyroid axis so slow?
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
Which thyroid dysfunctions would lead to development of a goiter?
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)
Major characteristics of hypothyroidism.
-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
major characteristics of hyperthyroidism
- 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
what would anti-thyroid drugs target in treatment of hyperthyroidism?
drugs block the uptake of Iodine by blocking the symporter uptake of Iodine, or drugs could inhibit TPO
Key characteristic of Graves’ disease is _____
exophthalmos
Why is T3 more potent than T4?
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.
TSH acts by increasing _____ in the thyrotropes.
cAMP
