Thyroid Flashcards
What does thyroid hormone do?
Controls rates of growth and metabolism
Thermogenesis/maintenance of core body temp
Iodine is used only by thyroid gland. What does the thyroid gland do to iodine?
Concentrates it and incorporates it into thyroid hormone T3 and T4
What clinical conditions occur if you have an iodine dietary excess?
Wolff-Chaikoff phenomena - reduction in TH levels caused by ingestion of large amount of iodine
Jod-Basedow phenomena - Hyperthyroidism after administration of iodine
Does not occur in people with normal thyroid glands
What is the purpose of a trophic hormone?
Nutrition - pituitary hormones exert nutrifying effects on target hormones - proper uptake of energy
What is the purpose of tropic hormones?
Directs synthesis and release of hormone of target organs
What are the two cellular signaling mechanisms of TSH action?
Gs –> Adenyl cyclase –> cAMP
Phospholipase C –> IP3 –> Ca2+
What is the effect of TSH on thyroid follicular cells?
Tropic effect - expression and regulation of every step in T4 synthesis and secretion
Trophic effect - can be seen in goiter
Which thyroid hormone is responsible for negative feedback?
T3 - this is the active form of TH
How many iodine atoms does thyroxine have?
4 - aka T4
How many iodine atoms does triiodothronine have?
3 - aka T3
How does epithelia of follicular cells change from an inactive thyroid state to an active thyroid state?
Cuboid –> Columnar
What do parafollicular cells do in the thyroid?
Calcium metabolism
What molecule is the colloid of the thyroid follicular cells rich in? What is it synthesized by?
Thyroglobulin - ER of follicular cell
Are T1 and T2 secreted?
No they are recycled until T3 and T4 are made
What hormone regulates synthesis of TH?
TSH
How does TH circulate in blood? What form of TH circulates?
99.7% bound to protein - TBG, albumin
T4 circulates
What does end organ cell do if it needs T3? What if it doesn’t need T3?
Deiodinates circulating T4
Doesn’t need it - turns it into inactive (reverse) T3
What are the effects of TH?
- Brown adipose tissue –> synthesis of UCP-1 leading to thermogenesis
- Reproduction –> Follicle dev, spermatogenesis, sex-hormone binding globulin (SHBG)
- Bone –> Maturation of chondrocytes, endochondral ossification, linear growth
- Muscle –> Creatine phosphate, bone remodeling
- Tooth and hair follicle dev
- Subcutaneous –> Inhibition of fibronectin synthesis
- CNS –> brain growth, in utero (growth of cortices, prolif of axons and dendrites), in infancy (increased T3 receptors), in adults (mental alertness, memory, reflexes, emotions)
- CV –> Increased β-adrenergic receptors, so increased heart rate and contractility (tachycardia)
How does fetus get TH?
Depends on maternal TH traveling across placental barrier to influence fetus
What causes cretinism? Symptoms?
Mother had no TH, so child has abnormal growth.
Muscular flaccidity, CNS retardation, large tongue, hernia b/c of weak muscles
What are some signs of hypothyroidism?
Low intelligence, delayed puberty, deep voice, weight gain
What are some signs of hyperthyroidism?
Rapid speech and reflexes, swelling of goiter
What is Grave’s disease? What would levels of TSH look like in someone with Grave’s?
Hyperthyroidism - autoimmune disease where Abs mimic TSH and bind to receptors so you make a huge amt of TH
TSH would be near zero b/c of so much neg. feedback
What is secondary hypothyroidism?
Issues with pituitary - no TSH is secreted even if patient is injected with TRH
