2003A15 Describe the physiological actions of thyroid hormones.

 

List:

·       Intro

·       Metabolic effects

·       Organ system effects

 

Intro:

Structure

·  Polypeptide

·  Contains iodine

Synthesis

·  In follicular cells

Axis

 

 

Thyroid hormone comparison

·  93% T4, 1x activity

·  7% T3, 3-5x activity

·  rT3 inactive

Mechanism of action

·  Multiple receptor subtypes

·  Receptors are nuclear transcription factor, binds THRE

·  ↑ or ↓ gene expression

Time course

·  Onset of action: 6-8 hours IV, 3-5 days oral

·  Duration of effect: T3 1 day, T4 7 days

o Longer if hypothyroid

o Shorter if hyperthyroid

 

Metabolic effects:

Overall

·  Effects are dose-dependent

·  ↑Metabolic rate (max 2x)

o ↑Na+K+ATPase activity

o ↑Catecholamine release

o Permissive effects

·  Exempt tissues: uterus, testes, spleen, anterior pituitary

Liver

·  ↑Glycolysis, glycogenolysis, gluconeogenesis, lipolysis

·  Protein: synthesis if ↔thyroid, breakdown and myopathy if ↑thyroid

Muscle

·  ↑Glycogenolysis, glycolysis

·  Protein: synthesis if ↔thyroid, breakdown if ↑thyroid (-> wasting)

Adipose

·  ↑Lipolysis -> fat loss

Bone

·  ↑Formation if ↔thyroid, ↑breakdown if ↑thyroid (-> osteoporosis)

GIT

·  ↑Carbohydrate absorption

 

Organ system effects:

Cardiovascular

·  ↑HR, CO, ↓SVR, MvO2, CBF (BP may be unchanged)

o Metabolic autoregulation -> vasodilatation -> ↑venous return

o ↑Catecholamines

o Permissive effect on catecholamines

o ↑Number of myocardial β1 adrenoceptors

Respiratory

·  ↑RR and TV

o ↑Metabolic rate -> ↑VCO2 (Krebs cycle)

CNS

·  In utero:

o Necessary for normal brain growth and development

·  In adults: ↑thyroid ->

o Anxiety, restlessness, tremor (brain)

o Hyperreflexia (↑spinal cord excitability)

Other

·  ↓Thyroid -> dry skin, thinning hair

·  ↑Thyroid -> proptosis, ↑LPS activity, thyroid stare