Lecture 11 3/25/25 Flashcards
What is the location of the thyroid gland?
2 mid-cervical lobes with extensive vascular supply
What is the functional unit of the thyroid gland?
follicle
What is a thyroid gland follicle?
sphere of cells with lumen containing colloid
What is the function of colloid?
primarily contains thyroglobulin, the reservoir for thyroid hormone
What are the characteristics of parafollicular cells?
-between the follicles
-synthesize and secrete calcitonin
What are the controls in place for the thyroid gland?
-hypothalamic-pituitary-thyroid axis
-autoregulation via iodine
What are the steps to thyroid hormone synthesis?
-thyroglobulin is synthesized and discharged to follicular lumen
-active iodine uptake, oxidation, and transport to follicular lumen
-iodine in lumen attaches to tyrosine in colloid to form MIT and DIT
-iodinated tyrosine are coupled together to form T3 and T4
-thyroglobulin of T4 and T3 are endocytosed into follicle cell
-T4 and T3 are released from lysosome into cytoplasm
-T4 and T3 diffuse into bloodstream
-cytoplasmic MIT and DIT are de-iodinated and recycled
What is the hypothalamic-pituitary-thyroid axis feedback pathway?
-hypothalamus produces TRH to stim. pituitary
-pituitary produces TSH to stim. thyroid gland
-thyroid gland produces T3, rT3, and T4
-thyroid gland products have negative feedback on hypothalamus and pituitary
What is the role of T4?
major regulatory product of thyroid
Which proteins bind to thyroid hormones?
-thyroxine-binding protein
-thyroxine-binding prealbumin
-albumin
-plasma lipoproteins
Why is it important that thyroid hormones are highly protein bound?
only the free/unbound portion of hormones produce biologic effects and/or provide feedback to the pituitary
How do the thyroid hormones interact with cells?
-thyroid hormones enter cells via transport proteins
-thyroid hormones must enter nucleus and bind to nuclear receptors to have effects
-fT3 enters cells more rapidly and has a more rapid onset than fT4
What are the most commonly measured hormones relating to the thyroid?
-total T4
-free T4
-thyroid stimulating hormone/TSH
Which systems are affected by thyroid hormones as part of their “multi-systemic hormone” function?
-fetal development
-heart rate and contractility
-lipid and carbohydrate metabolism
-respiratory center function
-erythropoiesis
-bone turnover
What is the normal total T4 range for most dog and cat breeds?
1.0 to 4.0
Which thyroid conditions most commonly occur in dogs and cats?
dogs: (primary) hypothyroidism
cats: (primary) hyperthyroidism
What is the impact of early non-thyroidal illness?
-decreased total T3
-decreased total T4
What is the impact of late and/or severe non-thyroidal illness?
-decreased total T3
-decreased total T4
-decreased free T4
-decreased TSH
What are the hormone levels seen in animals recovering from non-thyroidal illness?
-decreased total T3
decreased total T4
-increased TSH
What are the potential consequences of non-thyroidal illness affecting the thyroid hormones?
-false dx of hypothyroidism in dogs that are NOT hypothyroid
-false dx of “normal” in cats that ARE hyperthyroid
What are the key points to consider regarding hormone measurement for thyroid conditions?
-eliminate concurrent illnesses before testing when possible
-measure multiple hormones for improved diagnostic accuracy
