Thyroid Gland physiology Flashcards
What hormones does the thyroid gland produce?
Active T3 (triiodothyronine) and inactive T4 (tetraiodothyronine)
Describe the structure of the thyroid gland:
Thyroid follicle are surrounded by epithelial cells. Lumen in the center where thyroid hormones are stored in the colloid.
Also has parafollicular C cells that secrete calcitonin
What is the back bone of the thyroid hormone?
Thyroglobulin
What molecule is important for thyroid hormone synthesis?
Iodine (I2) from diet converted into I- and brought into thyroid gland via the iodide trap. Attached to the thyroid hormone
Draw out the synthesis equation of T3 and T4. What is the major product of this pathway?
Inactive T4. Later converted to T3 when needed via peripheral conversion
Draw the conversion pathway of T4 to T3
Ok
Peripheral conversion of T4 to active T3 can be hindered by …
Fasting
Physical stress
Catabolic disease
Draw the synthesis pathway of T3 and T4 inside the thyroid follicle
Ok
What enzyme recycles Thyroglobulin and Iodine? What happens if this is deficient?
Intrathyroidal deiodinase. Deficiency mimics Iodine dietary deficiency.
If there is low Iodine available, what is the favored product of thyroid synthesis?
T3 (requires less iodine)
What transports Iodine into the lumen of the thyroid follicle?
What happens if there is a mutation in this transporter?
Pendrin (a chloride iodide pump). Mutation = hypothyroidism = goiter and hearing loss since cochlear pendrin is also mutated.
How does hypothyroidism cause goiter?
TSH keeps stimulating the thyroid trying to make it make thyroid hormones = hyperplasia
What is perchlorate and thiocyanate?
Anions that compete with iodine binding to transporter. Decreases synthesis of thyroid hormones
How does PTU disrupt thyroid hormone synthesis?
What effect does this cause?
Disrupts peroxidase (which combines Iodine with TG) Causes buildup of Iodine which inhibits further synthesis of hormoness (Wolff-Chalkoff effect)
How does radioactive iodine uptake relate to thyroid gland activity?
Hyperthyroidism = more iodine uptake Hypothyroidism = less iodine uptake
What are the main binding proteins for thyroid hormones in the bloodstream?
Thyroxine binding globulin (TBG, higher affinity for T4)
Transthyretin (in brain tissue)
Albumin (everything else)
All of these are formed in the liver
Predict the T4 and resin uptake in these conditions (remember the pattern) Hyperthyroidism Hypothyroidism High TBG Low TBG Hepatic failure Pregnancy
Ok
Draw the HPT axis
OK
What is the pathway of TSH action on the thyroid gland?
TSH > GPCR > Adenlyl Cyclase > cAMP > mediate lots of things that help with T3/T4 production as well as thyroid gland growth
What is the mechanism of action of thyroid hormones?
-Act like steroid hormones (nuclear receptor)
Functions of thyroid hormones:
Growth
Synapse plasticity
BMR increase
Increase in cardiac output and B-adrenergic receptors
How exactly do thyroid hormones increase basal metabolic rate?
Thyroid hormone increases activity of Na/K ATPase leading to increase of O2 consumption and heat production]
Effect of T4 (thyroxine) is usually long lasting
Effects of thyroid hormone on lipid metabolism:
- Increase fat mobilization and FA oxidation
- Blood cholesterol level inversely correlated with thyroid hormone levels
- needed for carotene conversion to Vitamin A (eye problems and jaundice in hypothyroid patients)
Describe the effect of thyroid hormones on cardiovascular system:
Thyroid > T4 > T3 > Na/K ATPase > increase tissue thermogenesis > decrease systemic vascular resistance > increase speed and force of cardiac contraction > increase cardiac output > increase blood volume