Thyroid Hormone Actions

Question 1

What type of receptor are thyroid hormone receptors?

Answer 1

Nuclear receptors that function as ligand-activated transcription factors

Have a much higher affinity for T3 rather than T4

Question 2

What combination of nuclear receptors exhibits the highest affinity for T3?

Answer 2

Heterodimers composed of THR plus a RXR

Question 3

How are genes activated by T3?

Answer 3

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

Question 4

How are genes repressed by T3?

Answer 4

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

Question 5

What is the role of thyroid hormone in postnatal growth?

Answer 5

Essential for normal linear growth

Synergistic effect with growth hormone

Affects bone maturation

Question 6

What is the effect of thyroid hormone on the nervous system?

Answer 6

Essential for nervous system development

Affects nervous system functions like learning, memory, responsiveness, etc.

Question 7

How is the basal metabolic rate measured?

Answer 7

Heat production or oxygen consumption

Question 8

How does thyroid hormone control the BMR?

Answer 8

Direct and indirect actions to cause an increase in cardiac output and respiratory rate

Increase the delivery of substrates for oxidation

Question 9

What are the cardiovascular effects of thyroid hormone?

Answer 9

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

Question 10

How do thyroid hormones potentiate catecholamine action?

Answer 10

Inducing B-adrenergic receptor upregulation

B-adrenergic blocking drugs are useful in treating hyperthyroid patients

Question 11

What is congenital hypothyroidism?

Answer 11

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

Question 12

What is primary hypothyroidism?

Answer 12

Failure of the thyroid

Question 13

What is Hashimoto thyroiditis?

Answer 13

Autoimmune disease characterized by the production of antibodies that react with proteins of the thyroid gland

Most common cause of primary hypothyroidism

Question 14

What is an endemic goiter?

Answer 14

Occurs in 3rd world areas where there is insufficient iodine

Question 15

What is secondary hypothyroidism?

Answer 15

Result of a hypofunctional pituitary; isolated TSH deficiency

Question 16

What is tertiary hypothyroidsim?

Answer 16

Rare; characterized by diminished TRH release; results from damage to the hypothalamus

Question 17

What are some characteristics of hypothyroidism?

Answer 17

Tiredness, weight gain, cramping, joint pain, carpal tunnel syndrome

Later symptoms include: slowed speech, facial puffiness and periorbital swelling

Question 18

What is a myxedema coma?

Answer 18

Life-threatening complication that may occur in patients with long-term, poorly controlled hypothyroidism

Characterized by a deterioration of the patient’s mental status

Question 19

What is thyrotoxicosis?

Answer 19

Clinical syndrome characterized by thyroid hormone excess

Symptoms are a result of:

Increased metabolic activity in responsive tissues

Increases sensitivity to catecholamines

Question 20

What is Graves Disease?

Answer 20

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)

Question 21

What are three non-autoimmune causes of thyrotoxicosis?

Answer 21

Toxic adenoma

Toxic multinodular goiter

TSH-secreting pituitary adenoma