Thyroid Physiology Flashcards
T3
Triiodothyronine
T4
Thyroxine
- more abundant
Thyroid hormones are composed of ____ and _____
Tyrosine; iodine
Functions of thyroid hormones
- increased metabolic rate
- protein, lipid, CHO
Thyroid anatomy
Bi-lobed gland in cervical neck
- vascular!
- parathyroid glands (2 per lobe) = calcium metabolism
What is the functional unit of the thyroid
Follicle
Follicular cells
Single layer of cuboidal columnar thyroid cells surrounding lumen
- lumen filled with colloid
Parafollicular cells
Secrete calcitonin
- calcium regulation
T3/T4 synthesis
Thyroglobulin synthesis
- iodide trapping in follicular cell
- iodide oxidation
- organification: adding I to tyrosine resides on TG
- coupling of iodinated tyrosine residues
- release of hormone
Thyroglobulin
Precursor protein
- stored in colloid
- synthesized in follicular cells
- very large!!!
- thyroid hormone synthesized on tyrosyl residues of TG
_____ is required for T3/T4 synthesis
Iodide
- absorbed from GIT
- excreted by kidney
- uptake by thyroid gland via iodide trapping and iodide pump (active)
- uptake by salivary gland
Organification of iodine to tyrosine residues
Iodine added to tyrosine
- monoiodotyrosine (MIT)
- diiodotyrosine (DIT)
Coupling of iodinated tyrosine residues
- DIT + DIT = T4
- MIT + DIT = T3
Release of hormone
- endocytosis of colloid
- proteases break bonds between TG and T3/T4
- released into cirulation
Thyroid peroxidase
- iodide oxidation
- organification
- coupling
- hyperthyroidism drugs inhibit thyroid peroxidase
___ is more active than ____
T3; T4
- T3 enters cells more rapidly
- T3 is more potent
How much T3 is made in thyroid?
20%
- rest from de-iodination of T4 in tissues
- deiodinases present in many cell types (liver, kidney, muscle)
Deiodinase
- T4 is deiodinated to active T3 or inactive reverse T3
- rT3 formed during times of illness, starvation
Protein-binding
Necessary for transport
- 99% T3 and T4 are protein bound
- binding affinity determines half life = more rapid degradation of unbound hormone
Proteins
- thyroxine-binding globulin
- albumin
Only the ______ is active
Free hormone
- lipophilic, enters cell
Drugs alter protein-binding
Usually do not alter free T4 concentrations
- may bind to same proteins, lowering protein-bound fraction = decrease of total T4
- increase binding protiens = increased TBG, increase overall T4, not free T4
Thyroid hormones in circulation
T4: 99% protein bound - free form is biologically active T3: 99% protein bound - free form is biologically active - reverse T3 has no biological activity
Thyroid hormones do not affect which tissues?
- lungs
- testes/uterus
- retinas
- lymph nodes/spleen
Metabolic effects of thyroid hormone
- increases metabolic rate via oxidative phosphorylation in mitochondria
- catabolic effects: lipolysis, protein, carb utilization, GI absorption
- increased GI motility
Cardiovascular effects of thyroid hormone
- stimulation of cardiac beta adrenergic receptors = increased HR and cardiac output
- increased GFR
- stimulates erythropoiesis
Neuromuscular effects of thyroid hormone
- increased mental alertness
- necessary for normal growth and neuronal development
Hypothalamus
Input from higher centers in brain, body
- thyrotropin releasing hormone = increased TSH release
Pituitary gland
Thyroid stimulating hormone
- binds TSH receptors in thyroid
- stimulates synthesis of T3, T4
Thyroid axis has _______
Negative feedback
- free thyroid hormone inhibits TRH and TSH secretion
- increased T4 leads to decreased TRH and TSH
TSH promotes
- iodine trapping
- colloid endocytosis
- thyroid cell hypertrophy
Goiter
Decreased thyroid hormone secretion –> decreased negative feedback –> increased TSH = thyroid cell hypertrophy
- rare in dogs, seen in cats
If a normal cat is given a large amount of T3, what should happen to the TRH, TSH, and T4 concentrations?
Should decrease TRH, TSH, and T4
