Thyroid Hormone Flashcards
where is the thyroid gland found?
either side of the trachea, just below the larynx
not normally palpable in cats and dogs
thyroid gland composed of?
of follicles divided into clusters by connective tissue
follicles are filled with?
filled with colloid (which stains pink due to its proteinaceous nature), the main storage site for thyroid hormones bound to thryoglobulin
primary physiological function of thyroid hormone?
normal development, physical and mental well-being
growth
metabolism (increases O2 consumption & thermogenesis)
normal development of CNS
cardiovascular stimulant
promotes target cell responsiveness to sympathetic nervous system
Normal gonadal function
biochemical actions of thyroid gland?
thermogenesis
carb metabolism
lipid metabolism
protein metabolism
what are the steps in the synthesis of thyroid hormone?
1) follicular cells produce thyroglobulin which is sent into the colloid
2) thyroid stimulating hormone (TSH) increases iodide uptake
3) Iodination of thyroglobulin occurs which produces Thyroxine (T4) and Triiodothyronine (T3) they are still attached to thyroglobulin atp
active follicle of thyroid gland?
stored thyroid hormones are being extracted from the lumen and converted into active thyroid hormones
describe the epithelial cells of an inactive follicle?
epithelial cells relatively flattened
describe the epithelial cells of an active follicle?
epithelial cells taller and notice white edge on histological slide - due to non staining vacuoles, called reabsorption lacunae
levels of TH when hypothyroidism is occurring?
suboptimal levels of TH
Clinical signs of hypothyroidism?
reduction in overall metabolic rate:
heat seeking
mental retardation, dullness, sluggish, fatigue
slow reflexes + response
weight gain
weak pulse, bradycardia
SKIN ISSUES:
bilateral symmetrical alopecia (non-pruritic)
skin thickening
clinical signs of hyperthyroidism?
increased body temp
polydipsia (thirst) and polyuria (incr urination)
weight loss despite appetite
elevated heart rate and workload
increased metabolic rate leading to loss of skeletal mass
what can untreated hyperthyroid cats develop?
can develop heart failure
what is a parenchyma?
it is the tissue of an animal organ (as a gland) which performs the work of the organ instead of just being supportive tissue
what is the role of connective tissue in the thyroid gland?
it supports the parenchyma and carries blood vessels, lymphatics and nerves
the thyroid gland is composed of follicles, describe these follicles:
each follicle is a sphere of varying size with a wall of a single layer of epithelial cells (follicular cells) involved in thyroid hormone synthesis and secretion
what are the follicles of the thyroid gland filled with?
filled with colloid - the main storage site for thyroid hormones bound to thyroglobulin
what colour does colloid stain?
it stains pink due to proteinaceous nature
what are C-cells and where are these located?
C-cells are located in the connective tissue between follicles and produce calcitonin (involved in calcium regulation)
describe the follicle wall of the follicles of the thyroid gland:
follicle wall is a single layer of cuboidal epithelial cells; lumen is filled with colloid
how is the thyroid hormone essential for normal development?
it is essential for normal development of body tissue and for physical and mental well-being
every tissue is affected (in)directly by thyroid hormone
how does the thyroid hormone affect growth?
thyroid stimulates growth hormone secretion and increases production of IGF-1 by the liver, promoting the synthesis of structural proteins and skeletal growth
what generalised effects does the thyroid hormone have?
activates nuclear transcription of large numbers of genes in virtually all cells of the body.
Allows for enzymes, structural and transport proteins to be synthesised, leading to an increase in functional activity throughout the body
how is the thyroid hormones involved in metabolism?
increases overall basal metabolic rate
regulates the rate of O2 consumption and energy expenditure at rest
increased metabolic activity leads to increased heat production
Involved in both synthesis and degradation of fat, carb and protein
how is the thyroid hormone involved in normal development of the nervous system?
adequate production of thyroid hormone is important during foetal life & early brain development. Involved in myelination, thus impacting axonal speed conduction
how does the thyroid hormone affect the cardiovascular system?
it increases blood flow and cardiac output and increases heart rate (related to thyroid hormone promoting target cell responsiveness to the sympathetic nervous system)
increased heat production also leads to peripheral vasodilation
how does the thyroid hormone help with normal gonadal functions?
important for the function of the testes and ovaries - involved in sperm production and the oestrus cycle
what is thermogenesis?
it increases heat production and oxygen consumption by increasing mitochondrial oxidative phosphorylation
what effect does the thyroid hormone have on carbohydrate metabolism?
it stimulates glycogenolysis in the liver and potentiates the effect of adrenaline
enhances intestinal glucose absorption
what effect does the thyroid hormone have on lipid metabolism?
stimulates the release of free fatty acids by increasing the activity of hormone-sensitive lipase in adipocytes
stimulates the oxidation of fatty acids
enhances hepatic conversion of cholesterol to bile acids
what effect does thyroid hormone have on protein metabolism?
stimulates the synthesis of proteins mainly involved in oxidative reactions
stimulates protein catabolism
what is protein catabolism?
it is the breakdown of proteins into absorbable monomers for further degradation or reassembly
how is the thyroid hormone regulated?
via the hypothalamic-pituitary-thyroid axis
where is the TRH produced and what does it do?
it is produced in the hypothalamus and it stimulates the anterior pituitary gland
what is TRH?
Thyrotropin-releasing hormone
What is TSH?
thyroid stimulating hormone
what produces the TSH?
produced by the anterior pituitary gland
What requires TSH? (thyroid stimulating hormone)
the release of thyroid hormone
what does TSH do?
it binds to membrane receptors of thyroid cells
required for thyroid hormone release (no TSH = no thyroid hormone)
regulated the no. and size of follicular cells
describe the negative feedback for regulating the thyroid hormone:
circulating free T3 and T4 hormones inhibit the hypothalamus and anterior pituitary
what temp promotes the release of thyroid hormone?
low temps
collectively, what are T3 and T4 hormone known as?
known as thyroid hormone
where do all steps involved in the synthesis of thyroid hormone take place?
they take place in the colloid and all things needed for the synthesis are also found in the colloid
how do follicular cells take up colloid?
by endocytosis
where do T3 and T4 need to be released from?
needs to be released from thyroglobulin
what is T4?
thyroxine
what is T3?
triiodothyronine
where is colloid contained?
it is contained in endosome - fuses with lysosome which contains enzyme which releases T3 and T4 from thyroglobulin
T3 and T4 are?
lipid soluble
if T3 and T4 are lipid soluble, what does this mean they can do?
they can diffuse through basal membrane of follicular cell into blood
where do thyroid hormones bind?
they bind to carrier proteins in blood for transport to target cells
free T3 and T4 are…
biologically active
what happens when a follicle is active?
stored thyroid hormones are being extracted from the lumen and converted into active thyroid hormones
what happens during the circulation of thyroid hormone?
mostly T4 secreted, some T3
T3 is more biologically active
Bulk of T3 is derived by deiodination of T4 in peripheral tissues
99.5% of circulating thyroid hormone is bound to?
bound to plasma proteins
what is the main carrier protein for transporting thyroid hormone?
thyroxine binding globulin
(TBG)
Apart from TBG, what else does thyroid hormone bind to?
some also binds to thyroxine binding pre albumin (TBPA) and albumin
what is TBG?
thyroxine binding globulin
TBPA?
Thyroxine binding pre albumin
what are the two types of hypothyroidism?
primary and secondary
describe the primary type hypothyroidism:
related to the thyroid gland itself (most common)
plasma TSH is high
what does primary hypothyroidism cause?
lymphocytic thyroiditis, Idiopathic atrophy of the thyroid gland
describe the secondary type of hypothyroidism?
related to the pituitary gland/hypothalamus
and plasma TSH is low
what are the results of hyperthyroidism?
excessive production of thyroid hormones (T3 and T4)
Leads to an increase in overall metabolic rate
Commonly seen in CATS
Typically associated w/ benign thyroid tumours that increase hormone production
Goitre in animals is what?
enlarged thyroid gland
what are the common causes of goitre in animals?
Idiopathic
Iodine deficiency
Iodine toxicity
Ingestion of goitrogenic plants
what is a goitre?
a lump of swelling at the front of the neck
