Thyroid Flashcards
Identify the amino acid that thyroxine is derived from, and list another hormone derived from this AA.
Tyrosine
Catecholamines (Epinepherine, Norepinepherine)
Draw a diagram to show how T4 production is regulated by the hypothalamus and the pituitary.
Describe where thyroxine is stored in the thyroid follicle.
Stored in the lumen of the follicle as a colloid
Define what a goitrogen is, and how it acts.
A goitrogen blocks the production of T3 and T4 by inhibiting iodine uptake.
Describe how thyroxine is stored
Iodine is incorporated into tyrosine to make a colloid containing T3 and T4. It is stored as a colloid in the lumen of the follicle.
Describe the sequence of events involved in secretion of T4 and T3
When the follicular epithelium is stimulated, the coilloid containing Thyroxine is pinocytosed.
Identify the active thyroid hormone, and indicate where most of this active thyroid hormone is produced.
Free, unbound T4 and T3 is biologically active, there is more T4 than T3, so you measure clinical levels of T4
At the tissue level, T4 is converted to T3 by deiodinases, which is the biologically active hormone
Describe the feedback relationships of T4 on the hypothalamus and pituitary.
Negative feedback on the AP and hypothalamus
Describe the effect of T4 on protein synthesis.
It stimulates protein synthesis in almost all tissues
List common symptoms of hypothyroidism in dogs and hyperthyroidism in cats
Hypothyroidism: Underactive thyroid(Dogs). Fatigue, weight gain, heat seeking, alopecia, rat tail. Disease of middle aged dogs.
Hyperthyroidism: Overactive thyroid (Cats): increased metabolic rate, enlargement of thyroid, weight loss, heat intolerance, exo opthalmia, coat change. Often caused by multinodular adenomatous goiter, seen in cats 10+ yrs old most frequently.
Describe how you distinguish hypothyroidism of pituitary vs hypothalamic origin
It is hard to differentiated primary and secondary, usually done with extensive testing, but regardless, the treatment is the same for all the different variations of hypothyroidism. Primary is most common and often caused by Idiopathic atrophy or lymphocytic thyroiditis.
Explain the most common and useful clinical procedures for evaluating thyroid function
Total T4 Assay is most common
Free T4 (more expensive test but slightly more accurate)
TSH assay if nessecary
NO TSH ASSAY AVAILABLE FOR FELINES
Discuss treatments for hypothyroidism
L-Thyroxine: T4 replacement to fix T3 and T4 levels. Given SID or BID.
Higher dosages given to dogs than humans b/c absorption and metabolism of T4 is different for both.
Discuss advantages and disadvantages of various treatments for hyperthyroidism.
Antithyroid drugs: Pill the cat every day, stressful, but effective. Not a cure, just management of the hormone levels.
Surgical Thyroidectomy: surgery to remove thyroid, stressful, sedation required, invasive but effective; have to supplement thyroid medication afterwards
Radioactive Iodine: I131 is given as a SQ injection, far less stressful for the animal, but animal is radioactive for several days afterwards, one time treatment that cures problem usually, effective, safest
In dogs, usually a neoplastic thyroid tumor, so Sx, chemo, and Iodine are all viable options
T4 and T3
Thyroid hormones. There is much more T4 than T3 released by the thyroid but T4 is converted to T3 at the tissue level, and T3 is the most abundant cellularly active form.
What is iodide?
Converted form of iodine that is absorbed by the thyroid gland, where it is then converted back to iodine for use in thyroxine synthesis.
Define: Thyroidectomy
Removal of the thyroid gland, used for canine hyperthyroid neoplasia especially, as they are often malignant.
What is PLO?
Pluronic Lecithin Organogel, method for admin MMI, applied to pinnae in attempt to reduce GI side effects or owner compliance issues of oral MMI, takes longer than oral but less side effects
What is L-Thyroxine?
T4 given BID to normalize T3 and T4 levels.
What is Propylthiouracil?
Similar to Methimazole but used 20 years ago, not discussed in lecture much.
What is Methimazole?
Blocks thyroid hormone production by inhibiting TPO enzyme. Very forgiving drug, euthyroid within 2-4 weeks usually. Side effects including GI upset.
Define: Antithyroid drugs
Drugs like methimazole and Carbimazole which block thyroid hormone production in hyperthyroid animals.
Define: Thyrotropin
(TSH): Thyroid Stimulating Hormone
What is the active form of T4- free or bound?
Free T4 is the active form of thyroxine.
Define: Hypothalamic pituitary thyroid axis
Consists of Hypothalamus and Pituitary gland connected by infundibulum
Define: Idiopathic atrophy of the thyroid gland
atrophy of the thyroid gland with an unknown cause
Define: Thyroid hormone resistance
no receptors in place for T3/T4
Define: Laron dwarfism
form of dwarfism characterized by Growth Hormone resistance
Cretinism
hypothyroidism that is present during growth and development; causes severe mental retardation, inhibition of growth, disproportionate development
What is colloid?
found in the lumen of follicles in the thyroid gland; thyroxine is stored here until stimulated by TSH to be pinocytosed into the cell and released into the blood stream
Define: deiodinase
present in almost all tissues; removes an iodine from T4 to convert it into T3
What is tyrosine?
amino acid that thyroxine is derived from
What is hypothyroidism?
underproduction of thyroid, most common is primary, commonly seen in dogs
Clinical signs: weight gain, thin flaky hair coat, lethargy
treat by supplementing thyroid
What is hyperthyroidism?
Overproduction of thyroid hormone
Classical clinical symptoms- ravenous appetite, PU/PD, weight loss, cats
Typically a primary endocrine disorder
What is basal metabolic rate?
rate of energy expenditure at rest
