Thyroid Hormone Actions Flashcards
What type of receptor are thyroid hormone receptors?
Nuclear receptors that function as ligand-activated transcription factors
Have a much higher affinity for T3 rather than T4
What combination of nuclear receptors exhibits the highest affinity for T3?
Heterodimers composed of THR plus a RXR
How are genes activated by T3?
In genes expressed in this fashion, the unbound receptor is typically bound to the response element (TRE) and acts as a repressor until T3 is present
How are genes repressed by T3?
Binding of T3 to the THR represses the transcription of genes that are active in the absence of T3
E.g. TRH receptor and TSH subunits
What is the role of thyroid hormone in postnatal growth?
Essential for normal linear growth
Synergistic effect with growth hormone
Affects bone maturation
What is the effect of thyroid hormone on the nervous system?
Essential for nervous system development
Affects nervous system functions like learning, memory, responsiveness, etc.
How is the basal metabolic rate measured?
Heat production or oxygen consumption
How does thyroid hormone control the BMR?
Direct and indirect actions to cause an increase in cardiac output and respiratory rate
Increase the delivery of substrates for oxidation
What are the cardiovascular effects of thyroid hormone?
Increase the force and rate of cardiac contraction by promoting transcription of proteins taht enhance cardiac function
T3 increases the sensitivity of the heart to catecholamine stimulation
How do thyroid hormones potentiate catecholamine action?
Inducing B-adrenergic receptor upregulation
B-adrenergic blocking drugs are useful in treating hyperthyroid patients
What is congenital hypothyroidism?
Occurs in infants born with little/no thyroid issue, inherited thyroid hormone/receptor defect, iodide deficiency
Results in mental and physical retardation if left untreated
What is primary hypothyroidism?
Failure of the thyroid
What is Hashimoto thyroiditis?
Autoimmune disease characterized by the production of antibodies that react with proteins of the thyroid gland
Most common cause of primary hypothyroidism
What is an endemic goiter?
Occurs in 3rd world areas where there is insufficient iodine
What is secondary hypothyroidism?
Result of a hypofunctional pituitary; isolated TSH deficiency
What is tertiary hypothyroidsim?
Rare; characterized by diminished TRH release; results from damage to the hypothalamus
What are some characteristics of hypothyroidism?
Tiredness, weight gain, cramping, joint pain, carpal tunnel syndrome
Later symptoms include: slowed speech, facial puffiness and periorbital swelling
What is a myxedema coma?
Life-threatening complication that may occur in patients with long-term, poorly controlled hypothyroidism
Characterized by a deterioration of the patient’s mental status
What is thyrotoxicosis?
Clinical syndrome characterized by thyroid hormone excess
Symptoms are a result of:
Increased metabolic activity in responsive tissues
Increases sensitivity to catecholamines
What is Graves Disease?
Most common cause of thyrotoxicosis
Autoimmune disease characterized by hyperplastic goiter
Thyroid-stimulating immunoglobulin that stimulates the TSH receptor
Exhibit thyroid-associated ophthalmopathy -periorbital edema and proptosis (anterior bulging of the eyeballs)
What are three non-autoimmune causes of thyrotoxicosis?
Toxic adenoma
Toxic multinodular goiter
TSH-secreting pituitary adenoma
