Thyroid Physiology Flashcards
Within the thyroid, follicular cells secrete __ hormones whereas chief cells secrete ______ hormones.
Follicular cells = T3/T4 hormones;
Parafollicular (C-) cells = Calcitonin hormone
Out of the produced hormones from the follicular cells, ___ is the precursor and ___ is the bioactive hormone.
T4 prehormone (91%); T3 active hormone (7%)
What is the significance of thyroid peroxidase (TPO) in the biosynthesis of thyroid hormone. Explain it in the 3 steps of thyroid hormone biosynthesis.
TPO first oxidizes tyrosine to the synthesized thyroglobulin protein. Secondly, TPO iodinizes the secreted thyroglobulin then lastly couples this protein to existing colloid in the follicular lumen.
What thyroid hormone is the most active form and from where is it formed?
T3 is the most active form that is made peripherally from circulating T4 hormone which is in abundance in the body.
What are the 3 main binding proteins for thyroid hormones?
Thyroxine-Binding Protein (TBG), Thransthyretin and Albumin are the 3 major binding proteins for T3/T4.
What effect does elevated levels of T3/T4 have on the Hypothalamic-Pituitary-Thyroid Axis (HPA)?
Rising thyroid hormone levels inhibit TRH and TSH production via negative feedback mechanism.
What effect does Thyroid stimulating hormone (TSH) have on metabolism and the thyroid?
This increases the basal metabolic rate (BMR), increases the proliferation of thyroid cells and increases vascularization within the gland. It stimulates ATP production over a long period of time.
Describe the pathogenesis of Hashimoto’s Thyroiditis.
This disease is hypothyroidism in which thyroid inhibiting immunoglobulins (TII antibodies) attach the thyroid gland leading to reduced TSH secretion and inflammation.
Describe the pathogenesis of Grave’s Disease.
This disease is associated with Hyperthyroidism. It involves thyroid stimulating immunoglobulins (TSI antibodies) that targets the thyroid gland resulting in high levels of T3/4 and TSI levels.
What is the correlation between TSH and thyroid hormones in Primary Hypothyroidism?
High TSH levels and low T3/4 hormones are indicated in Hashimoto’s Disease.
What is the correlation between TSH and thyroid hormones in Primary Hyperthyroidism?
Low TSH levels and high T3/4 hormones as seen in Grave’s Disease. TSH levels are low due to the negative feedback of high thyroid hormones.
The conversion of T4 to T3 hormone is catalyzed by the hormone: __________. The enzyme responsible for converting T4 to inactive rT3 is: _________.
5'-deiodinase = T4 to T3; 5-deiodinase = T4 to rT3
About 80% of circulating T4 hormone is bound to ________; whereas, the remaining 20% is bound to _____ and _____.
Thyroxine-binding protein (TBG); transthyretin and albumin
Thyroid hormone receptors alpha and beta are ______ in ______ tissues of the body. They are found on chromosomes ____ and ____, respectively.
TR-alpha = chromosome 17; TR-beta = chromosome 3; both are co-expressed in most tissues of the body.
Describe the mechanism by which thyroid hormone regulates body temperature.
T4 stimulates txn factors that upregulate mRNA which produces respiratory enzymes that stimulates mitochondria. A boatload of ATP is generated to which a majority of it is used up by the cell. The remaining energy is emitted as heat that is used for body temperature.