Endocrine Glands- Thyroid Flashcards
The thyroid gland produces Which types of hormones?
The thyroid produces thyroid hormone ( T3/T4) (follicular cells) and Calcitonin (parafollicular cells).
What are the two molecules that are important for synthesizing thyroid hormones?
Iodine and Thyroglobulin
How is iodide transported into the thyroid follicle to synthesize thyroid hormones?
Iodine is sourced from food -> transported in blood as iodide -> through active transporter (of Na+/I- symporter) iodide gets into the cell and than pendrin transfers it into lumen.
How are T3 and T4 synthesized?
Iodine is sourced from food -> transported in blood as iodide -> through active transporter (of Na+/I- symporter) iodide gets into the cell and than pendrin transfers it into lumen. -> Iodide and thyroglobulin will be oxidized via thyroid peroxidase -> it becomes diiodotyrosine or monoiodotyrosine -> 2 of these molecules get together ( if 2 diiodotyrosine get together than it is tetraiodothyrinine (T4) If 1 monoiodotyrosine and 1 diiodotyrosine get together than it is triiodothyronine (T3) -> it then is endocytosed, and undergoes proteolysis within the cell. It is lastly transported out of cell to binding proteins to be transported via blood.
How are T3 and T4 transported?
after being synthesized it then is endocytosed, and undergoes proteolysis within the cell. It is lastly transported out of cell to binding proteins to be transported via blood. It is lipid soluble and is transported in blood with plasma proteins. These include Thyroxine binding protein (TBG), Albumin, and Thyroxine- binding, prealbumin. Only free hormone is biologically attached.
How are thyroid hormones metabolized?
- Deiodination -> Remove iodine ( main form) Mediated by 5’deiodinase
- Formation of hormone conjugates: Sulfates and glucuronides
- Modification of alanine moiety of the thyronine ( mediated by transamination or decarboxylation)
How do thyroid hormones execute their biological functions?
Lipid soluble -> can penetrate cell membrane-> T3 goes to nucleus and is bound to T3 receptor -> complex is formed -> Complex binds to DNA sequence ( called response element) -> creates new proteins for biological function.
How is the synthesis of T3 and T4 regulated by the hypothalamus and pituitary gland?
Thyroid hormone synthesis is stimulated by TSH from the anterior pituitary gland. The anterior pituitary gland is given a message from the hypothalamus via TRH to secrete TSH. If there is too much TSH or TRH will cause negative feedback mechanism.
What are the common causes of hypothyroidism?
Primary:
- Lymphocytic thyroiditis -> inflammation destroys thyroid
- Congenital: Thyroid dysgenesis -> developed abnormally.
Secondary:
- Pituitary tumors -> Tumor
- Radiation Therapy -> decreased tissue function
- Ingestion of endogenous or exogenous glucocorticoids
( all of these are due to not enough pituitary hormone)
Tertiary:
- Hypothalamic tumors
- congenital as a result of TRH or TSH receptor defects
( these are because if deficit in thyrotropin release hormone.
What are the common clinical symptoms of hypothyroidism?
- Lethargy, obesity are the most common clinical signs. Difficult to diagnosis at first due to very mild symptoms in the beginning.
More pronounced symptoms ( later):
- Derm: Symmetrical truncal or tail head alopecia, thickened skin.
- Hair symptoms: Dull dry hair, poor hair regrowth after clipping. Puppy hair
- Cardiovascular signs (uncommon): Bradycardia, decreased cardiac contractility, atherosclerosis.
- Neuromuscular signs ( uncommon): myopathies, megaesophagus
- Neuropathies (uncommon): Bilateral or unilateral facial nerve paralysis, vestibular disease, lower motor neuron disorders
- Myxedema coma (unusual) : secondary to myxedematous fluid accumulations in brain and severe hyponatremia.
Others: Prolonged in-estrous intervals, silent heat, delivery of weak/ stillborn puppies, corneal lipid deposits, constipation.
How is hypothyroidism diagnosed?
- clinical signs,
- pathological findings ( anemia, hypercholesterol, hypernatremia, increased serum CPK. )
- Lab tests: decreased TT4 and TT3 concentrations. Reduced Free T4 (accuracy can be up to 90%), Increased TSH levels
- 100% accuracy doing all of these things
How is hypothyroidism treated?
Supplement thyroxine ( inexpensive) life long treatment.
How is hyperthyroidism diagnosed?
Elevated TT4, FT4 (can show elevated levels in early disease) T3 suppression test: Normal cats will have 50% suppression T4 due to negative feedback mechanisms. Diseased cats fail to respond to negative feedback mechanisms
How is hyperthyroidism treated?
Treatments:
- Sx ( difficult due to unknown qty to remove, scars, ect)
- Radioisotope iodine level -> will destroy thyroid and become hypothyroid.
- Medication: Methimazole
