Thyroid Flashcards
What are the three hormones secreted by the thyroid gland?
Triiodothyronine (T3),t1/2 1-2 dys (9%)
Thyroxine (T4), t1/2 6-7 dys (90%)
Calcitonin
Which is more potent T3 or T4?
T3 is 4x more potent than T4
Where is the majority of T3 produced?
Majority of T3 produced from peripheral conversion of T4 to T3
What medications inhibit the peripheral conversion of T4 to T3?
Inhibited by beta-blockers, corticosteroids, amiodarone
What is the biosynthesis of thyroid hormones?
Hypothalamus produces thyrotropin-releasing hormone (TRH)
Stimulates pituitary gland to synthesize and release thyroid stimulating hormone (TSH)
Circulating TSH stimulates thyroid gland
Concentrate iodine
Synthesize thyroid hormone
release thyroid hormone
Peroxidase enzymes catalyze iodination of tyrosine on thyroglobulin forming MIT and DIT
Coupling of 1 DIT and 1 MIT or 2 DIT, to form T3 and T4
What do cardiac glycosides do to iodine concentration?
Cardiac Glycosides-by inhibiting potassium accumulation can block iodide uptake
What do bromine, fluorine, and lithium do to iodine concentration?
Bromine, Fluorine, Lithium- block transport of iodide into thyroid
What is the ratio release of T3:T4?
Release ratio (T4:T3) = 4:1 (releasing more T4 than T3
Is T3 or T4 absorbed at a higher rate and which is more active?
T3 absorbed at higher rate, more active
How is T3 transported into the plasma?
Thyroid Binding Globulin (TBG)
Albumin
Thyroid Binding PreAlbumin (TBPA)
What percentage of thyroid hormone is protein bound?
99%
What is the only portion of hormone that is available to elicit biological effect and regulate TSH?
Free hormone
What are the effects of thyroid hormone?
- Determination of basal metabolic rate
- Influence of growth through stimulation of growth hormone synthesis and action
- Body temp
- Fetal development
- Cardiac rate and contractility
- Peripheral vasodilatation
- Red cell mass and circulatory volume
- Respiratory drive
- Peripheral nerves (reflexes)
- Hepatic metabolic enzymes
- Bone turnover
- Skin and soft tissue effects
What is a clinical and biochemical syndrome resulting from decreased thyroid hormone?
Hypothyroidism
What is a clinical and biochemical syndrome resulting from increased thyroid hormone?
Hyperthyroidism
What labs should be assessed for thyroid dysfunction?
Total T4- free and bound
Free T4 (FT4)- more reliable than total T4 and T3 levels
Sensitive TSH- evaluates the negative feedback system
What is TSH increased in?
Increased in primary hypothyroidism (something is wrong with the thyroid its not releasing enough thyroid hormone and its feedback to the pituitary saying to release TSH)
What is sensitive TSH decreased in?
Decreased in primary hyperthyroidism (thyroid is producing thyroid hormone and sending feedback to pituitary saying not to release TSH)
What are the thyroid function test results with hyperthyroid?
Total T4- Elevated
Free T4- Elevated
Total T3- Elevated
TSH- LOW
What are the thyroid function test results with hypothyroid?
Total T4- low
Free T4- low
Total T3- low
TSH- ELEVATED
What are the increased thyroid binding globulin thyroid function test results?
Total T4- Elevated
Free T4- Normal
Total T3- Elevated
TSH- Normal
What is a serum TSH concentration above statistically defined upper limit of reference range?
Sub-clinical hypothyroidism
What are the CV effects of sub-clinical hypothyroidism?
Increased risk of coronary heart disease
What are the fertility and pregnancy effects of sub-clinical hypothyroidism?
Placental abruption risk 3x higher
Preterm delivery risk 2x higher
Women with thyroid autoantibodies have increased risk for abortion