Thyroid Function Flashcards
What are the two types of cells in the thyroid gland?
Follicular and parafollicular
What thyroid hormones do follicular cells make and secrete?
T4 (L-thyroxine)
T3 (L-triiodothyronine)
rT3 (reverse T3 (biologically inactive))
Where are hormones stored in follicular cells?
Lumina of the follicle
What hormones do parafollicular cells secrete?
Calcitonin (involved in calcium regulation)
List the functions that thyroid hormones are involved in
Rate of O2 consumption
Growth
Sexual maturity
Protein and carbohydrate metabolism
TRH
Released by hypothalamus and stimulates anterior pituitary to secrete TSH
TSH
Regulates synthesis and release of the thyroid hormones
What regulates the secretion of TSH?
TRH
somatostatin: inhibitory factor
FT3 and FT4: stimulate hypothalamus to secrete somatostatin. Exert neg feedback to anterior pituitary to inhibit TSH secretion
H-P-T axis
The negative feedback loop of TRH and TSH
Thyroid hormones circulate the blood bound to what?
Thyroxine-binding globulin (TBG)
Thyroxine-binding prealbumin
Thyroxine-binding albumin
T/F:
Biologically inactive = physiologically active
TRUE
What is the function of TBG?
Principle carrier protein
FT3 and FT4 are physiologically ___
Active
T/F:
T3 is 4-5 times more metabolically potent than T4
TRUE
What is the most useful test for assessing thyroid function
TSH
Characteristics of hypothyroidism
TSH is minimally increased
FT4 stays within normal range
Characteristics of subclinical hyperthyroidism
TSH is low
FT4 is normal
T/F:
Up to 90% of thyroid hormone are protein bound
FALSE
MORE than 99.9% of thyroid protein are protein bound
T/F:
alteration in thyroid hormone binding proteins frequently lead to total T4 outside of normal range without representing true clinical thyroid dysfunction
TRUE
Thyroid produces approximately ___% of T3
20%
T3 contains 3 atoms of ___
Iodine
T/F:
FT4 makes up a large fraction of total thyroxine
FALSE
Small fraction because most are bound to thyroid carrier proteins