Physiology Block 3 Week 14 10 Thyroid Hormone Flashcards Preview

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Flashcards in Physiology Block 3 Week 14 10 Thyroid Hormone Deck (16)

Anatomy of Thyroid Gland

-balls consisting of a shell made of cells filled with colloid

Parafollicular cells:
-synthesize calcitonin


Amino Acid Precursor of Thyroid Hormone Synthesis

What is added?


Addition of iodine

Monoiodotyrosine (MIT)
T3: Triiodothyronine (MIT + DIT)
T4 Thyroxine (DIT + DIT)


What happens to the iodine absorbed in the small intestine from the diet?

3/4 excreted into the urine

1/4 of the iodide is pumped into the thyroid follicular cells via iodide pump
--60% organified into thyroid hormones
--40% leaks back out into plasma (urine excretion)

Of the iodide circulating part of thyroid hormone (T3 & T4), 80% is de-organified in the tissues and excreted in the urine
--remainder eliminated in feces

97% of absorbed iodide is eliminated in urine


Thyroid Hormone Synthesis

1. Iodide pumped into follicular cell with Na+ via NIS symporter due Na/K ATPase gradient

2. Thyroglobulin (Tg) synthesized within follicular cell and secreted into colloid space

3. Iodide is pumped into colloid and exchanged with chloride via PENDRIN transporter
--converted into I2

4. Peroxidase
-produces peroxide that act on I2 to make it iodinate tyrosines on the Tg molecules
-catalyzes coupling to form T3 and T4

5. TSH stimulates pinocytosis of Tg in colloid
-brings Tg into intraccellular space as colloid droplets
6. Proteases act on colloid droplets, liberating MIT, DIT, T3, and T4 from Tg

7. T4 and T3 secreted into EC space; MIT and DIT recycled


T4 half-life

T4 bound to thyroid binding globulin (TBG), transthyretin, and loosely to albumin

<0.05% circulates in free form

Very long half-life


T3 half-life

T3 does NOT bind transthyretin
--circulates a little less tightly bound to plasma proteins than T4 (also more potent than T4)


Neonatal (Congenital) Hypothyroidism

What is the screening test?

Bone and Height age are very low before treatment

Mental age is very low and NOT improved by treatment
--CNS development complete by 5
--treatment needs to start at birth

Screening test:


Effects of Thyroid Hormone

Activate transcription and translation of new proteins


CNS Development

Metabolic effects:--increasing oxygen consumption in mitochondria leading to an increased basal metabolic rate

Cardiovascular (and Respiratory): in order to supply substrate and oxygen to the cells for increased metabolism, cardiac output and alveolar ventilation are increased


Effect of Thyroid Hormone on Blood Pressure Regulation

Thyroid hormones are sympathomimetic
--do not directly increase sympathetic nerve activity
--augment sensitivity to catecholamines and sympathetic input to tissues

**Increased beta adrenergic receptor activity**

Excess adrenergic activity = tachycardia


Control of Thyroid Function

Thyroid Stimulating Hormone (TSH) regulates Thyroid Function

-iodine uptake
-TH synthesis and secretion (increases metabolism and negative feedback on pituitary)

Increased TH inhibits TSH release

Thyroid Releasing Hormone probably laters negative feedback sensitivity of pituitary
-increased TRH induces an inc in circulating TH


Primary Hypothyroidism

Hashimoto's Thyroiditis

Low Free T4
High TSH

Autoimmune attack on the thyroid gland
--decrease in function

Loss of negative feedback
Not enough T4 to inhibit TSH release


Secondary Hypothyroidism


Low Free T4
TSH NOT elevated

Inappropriately low TSH secretion for level of fT4


Primary Hyperthyroidism

Graves' Disease

High fT4

Autoimmune disease--thyroid-stimulating immunoglobins are produced that activate TSH receptor, increasing fT4

High fT4 suppress TSH release


Why is TBG elevated in pregnant women?

Effect of Estrogen or estrogen therapy

Total T4 will be elevated because there is more binding protein

Free T4 will be normal because pituitary is normal and results in normal negative feedback of TSH (normal levels)


What would TSH be in the typical patient with hypothyroidism due to hypopituitarism?

Low fT4
Non elevated TSH due to hypopituitary function


Which of the following patients will have increased serum TSH levels?

Graves' Disease
Excessive iodized salt intake
Hashomoto's Thyroiditis
Excessive levothyroxine (T4) therapy

Hashomoto's Thyroiditis

Primary hypothyroidism
Low fT4
High TSH

Thyroid function decreased resulting in decreased T4 made leading to loss of negative feedback on TSH