Thyroid Gland Physiology Flashcards
Which enzyme converts T4 –> T3 or reverse T3 in the periphery?
- T4 –> T3 through outer ring deiodination via deiodinases type 1 and 2
- T4 –> reverse T3 through inner ring deiodination via deiodinase type 3
Which clinical states are associated with reduction in conversion of T4 –> T3?
- Fasting
- Medical and surgical stress
- Catabolic diseases
Which enzyme liberates the I- from recycled MIT and DIT to be reused within the thyroid follicular cell?
- Intrathyroidal deiodinase
- Deficiency of this enzyme mimics dietary I- deficiency
Pendred syndrome
- Mutation in the Pendrin gene (SLC26A4 aka PDS) causing defects in the transport of I-/Cl- across the apical membrane of the thyroid follicular cells
- Also impacts the cochlea
- Pt presents w/ hypothyroidism, goiter and sensorineural hearing loss
Describe the Wolff-Chaikoff effect
-High circulating lvls of I- transiently inhibit the organification and synthesis of thyroid hormones in an attempt to prevent the thyroid from synthesizing large quantities of TH
Perchlorate and thiocyanate
- Competitive blockers for the Na+/I- synporter on the basolateral membrane of the thyroid follicular cell
- Used to decrease thyroid hormone synthesis in hyperthyroidism pts
Propylthiouracil (PTU)
- Inhibits thyroid peroxidase (PTO) activity
- Used to decrease thyroid hormone synthesis in hyperthyroidism pts
Expected values of total T4 and T3 resin uptake compared to normal in pts w/: Hyperthyroidism
- total T4: increased
- T3 resin uptake: increased (more T4 competing for same # of binding spots on TBG)
Expected values of total T4 and T3 resin uptake compared to normal in pts w/: Hypothyroidism
- total T4: decreased
- T3 resin uptake: decreased (less T4 competing for same # of binding spots on TBG)
Expected values of total T4 and T3 resin uptake compared to normal in pts w/: high TBG
- total T4: increased
- T3 resin uptake: decreased
Expected values of total T4 and T3 resin uptake compared to normal in pts w/: low TBG
- total T4: decreased
- T3 resin uptake: increased
Expected values of TBG and T3 resin uptake compared to normal in pts w/: hepatic failure
- TBG: decreased –> transient increase in free T3, T4 which inhibits synthesis of T3, T4 resulting in
- T3 resin uptake: increased
Expected values of TBG and T3 resin uptake compared to normal in pts w/: pregnancy
- TBG: increased due to excess estrogen production
- increased bound T3 and T4 w/ decrease in free T3 and T4 –> transient decrease in free T3, T4 lvls causes an increase in synthesis and secretion of T3, T4 –> increase in total T3 and T4 lvls but amount of free T3, T4 are normal (clinically euthyroid)
- T3 resin uptake: decreased
Actions of thyroid hormones on lipid metabolism
- Stimulates fat mobilization –> increase [FA] in plasma
- Enhances oxidation of FA
- Plasma [cholesterol] and [triglycerides] inversely correlated w/ thyroid hormones –> increased blood [cholesterol] in hypothyroidism
- Required for conversion of carotene to vitamin A (hypothyroid pts can suffer from blindness and yellowing of skin)
Actions of thyroid hormones on carbohydrate metabolism
- Increased gluconeogenesis and glycogenolysis to generate free glucose
- Enhanced insulin-dependent entry of glucose into cells
Cardiovascular effects of thyroid hormones
Main goal is to increase cardiac rate/output to increase blood volume:
- Direct:
a) increases cardiac muscle myosin heavy chain alpha/beta ratio
b) stimulates synthesis of cardiac Beta1-adrenergic receptors
b) increases ventricular contractility and function
c) decrease peripheral vascular resistance - Indirect: increase heat production and CO2 in tissues –> decrease peripheral vascular resistance –> decrease diastolic BP –> reflex increase adrenergic stimulation
Actions of thyroid hormones on growth
-Act synergistically w/ growth hormone and someatomedins to promote bone formation
Actions of thyroid hormones on CNS
- important for CNS maturation and deficiency during perinatal period leads to:
a. abnormal development of synapses
b. decreased dendritic branching and myelination
Major clinical signs and lab findings in Graves’ disease
- Exophthalmos and periorbital edema
- Elevated serum free and total T4 or T3 lvls and decreased TSH lvls w/ signs of goiter and ophthalmopathy
- Presence of circulating thyroid-stimulating immunoglobulins distinguish Graves’ disease from adenoma of pituitary thyrotrophs
Hashimoto’s thyroiditis
- primary hypothyroidism caused by production of thyroglobulin or thyroid peroxidase antibodies which attack the thyroid gland
- Labs will show decreased T3 and T4 with high TSH lvls and clinical signs of goiter due to TSH
Sheehan syndrome
- postpartum hypopituitarism secondary to necrosis of pituitary gland –> caused by acute hemorrhage during child birth that resulted in a rapid drop of BP which shunted blood away from brain
- Most pts present w/ agalactorrhea
- Can also see amenorrhea and hypothyroidism (secondary disorder)
