Physiology: Physiology of the Thyroid Flashcards
Describe the effect of thyroid hormones on lipid metabolism
Mobilised fats from adipose tissues, increased fatty acid oxidation in tissues
Name three states which increase thyroxine binding globulin levels
Pregnancy, chronic active hepatitis, billiary cirrhosis
Which type of de-iodinase breaks down the majority of thyroid hormones?
Type III (D3)
Describe the effect of thyroid hormones on growth
Growth hormone releasing hormone (GHRH) production and secretion, and glucocorticoid-induced GHRH release are dependent on thyroid hormones
Describe the effect of thyroid hormones on metabolic rate
Increase basal metabolic rate
Where are Type II (D2) de-iodinases found?
Heart, skeletal muscle, fat, thyroid, and pituitary
Describe the effect of thyroid hormones on the development of foetal and neonatal brain
Myelinogenesis and axonal growth require thyroid hormones
Describe the effect of thyroid hormones on the lungs
Increased breathing rate
What affect do T3 and T4 have on the hypothalamus and anterior pituitary?
Suppress the production of TRH and TSH - complete the negative feedback loop
Describe the structure of a thyroid follicle
Colloid surrounded by cuboidal epithelial cels and a basement membrane
Name the organ which produces TSH
Anterior pituitary
Name the principle thyroid hormone binding protein
Thyroxine binding globulin (70%)
How are the thyroid hormones formed from the colloid?
Iodine moves into the colloid and attaches to tyrosine residues on thyroglobulin to form DIT (T3) and MIT (T4)
Name three states which decrease thyroxine binding globulin levels
Large doses of glucocorticoids/Cushing’s, severe systemic illness, chronic liver disease
Which thyroid hormone acts as the pro hormone?
T4 - converted to T3 by liver and kidney to become biologically active
How do thyroid hormones increase metabolic rate?
Increase number and size of mitochondria, increase oxygen use and rates of ATP hydrolysis, increase synthesis of respiratory chain enzymes
Why do most labs measure free T3/4 rather than serum levels?
Metabolic state correlates more closely with the free than with the total concentration of thyroid hormones in the plasma
Describe the effect of thyroid hormones on liver tissue
Increased glycogenolysis and gluconeogenesis, decreased glycogenesis
What is the colloid in the thyroid follicle?
Iodinated glycoprotein thyroglobulin
Describe the effect of thyroid hormones on the heart
Increased HR and force of contraction
Which thyroid hormone accounts for ~90% of thyroid hormones secreted?
T4 (thyroxine)
What percentage of thyroid hormones are bound to hormone-binding proteins?
More than 99%
Describe the effect of thyroid hormones on the central nervous system
Hypothyroidism - slow intellectual functions
Hyperthyroidism - nervousness, hyperkinesis and emotional lability
What do the parafollicular cells secrete?
Containing calcitonin-secreting C-cells
Name the organ which produces TRH
Hypothalamus
Why does thyroid hormone increase breathing rate, heart rate and force of cardiac contraction?
Helps meet increased O2 demand
Describe the secretion of T3 and T4
Colloid is enveloped by microvilli on cell surface to form colloid vesicles which fuse with lysosomes; enzymes break down the lysosomes releasing thyroid hormone
Which thyroid hormone accounts for ~10% of thyroid hormones secreted?
T3 (triodothyronine)
Name the key enzymes involved in degradation of thyroid hormones
De-iodinases
What is the function of TSH?
Binds to TSH receptor (GPCR) on surface of thyroid epithelial cells and via cAMP increases production and release of thyroid hormones (T3 + T4)
Describe the effect of thyroid hormones common to all cells but especially neurones
Increase metabolic rate, increased glucose uptake
Where are Type III (D3) de-iodinases found?
Faetal tissue, placenta, and brain (except pituitary)
Where are Type I (D1) de-iodinases found?
Liver and kidney
What is the function of TRH?
Stimulates anterior pituitary to produce TSH
Name the three types of hormone-binding proteins which thyroid hormones bind to
Thyroxine binding globulin, thyroxine binding pre-albumin, albumin
Describe the permissive sympathomimetic action of thyroid hormones
Thyroid hormones increase responsiveness of adrenaline and noradrenaline by increasing the number of receptors
Which thyroid hormone is the most potent?
T3 (triodothyronine)
Describe the effect of thyroid hormones on adipose connective tissue
Increased lipolysis, decreased lipogenesis
How do free thyroid hormones increase BMR of target cells?
Bind to receptor on target cells, translocate into the nucleus and bind to thyroid response elements on target genes to stimulate transcription of these genes
Describe the effect of thyroid hormones on carbohydrate metabolism
Increased blood glucose due to stimulation of glycogenolysis and gluconeogenesis, increased insulin-dependent glucose uptake into cells
Describe the effect of thyroid hormones on protein metabolism
Increased protein synthesis
