Day 4 Thyroid and Pineal Gland, Thyroid Hormone Synthesis

When is the pineal gland most active?

at night/when dark

What is secreted by the pineal gland?

melatonin

What is the role of melatonin?

maintains circadian rhythm (sleep wake cycle)

The thyroid is found where?

first or second tracheal ring

The two lobes of the thyroid gland are connected by a narrow piece of tissue called what?

isthmus

The functional unit, or glandular tissue of the thyroid gland is known as?

follicle

Cells are arranged in what way within the follicle?

circular

The substance that fills the follicles is known as

colloid

What is the main storage form of the thyroid hormones?

TGB in colloid

What is stored in the colloid?

TGB produced by follicular cells

What is the function of the parafollicular cell (c-cell) of the thyroid?

secretes calcitonin

90% of the thyroid hormones produced in the follicle are what?

T4

Which thyroid hormone is more potent?

T3 (4x more potent)

What hormone is important for the regulation of calcium?

calcitonin

What molecules are important for thyroid hormone synthesis?

tyrosine and iodine

If one iodide molecule binds to the tyrosyl ring, it is called a

monoiodotyrosine

Coupling of two iodide molecules to the tyrosyl ring is called

diiodotyrosine

Coupling of two iodinated tyrosine molecules results in the formation of what?

thyroid hormones

Two diiodotyrosine molecules forms what?

T4

One monoiodotyrosine and one diiodotyrosine molecule forms what?

T3

A key enzyme in the biosynthesis of thyroid hormones is what?

thyroperoxidase

What does thyroperoxidase do?

oxidizes iodide to iodine in the follicular cell

How does thyroperoxidase aid in the synthesis of thyroid hormones?

catalyzes the iodination of tyrosyl residues of TGB

What role does thyroglobulin play in thyroid hormone synthesis?

substrate for synthesis and storage of inactive forms

Thyroid hormones are the only hormones that contain what?

halide (iodine)

When thyroid hormones are synthesized, they remain in the _______________________ until release.

extracellular acinar lumen

The unique extracellular storage of thyroid hormones allows for what?

large hormone reserve

Majority of T3 formation occurs outside the thyroid gland by what process?

deiodination of T4

What tissues have the highest concentrations of deiodinating enzymes?

liver and kidney

What is the most important carrier protein for thyroid hormones?

TBG

TBG has high affinity for which thyroid hormone?

T4

TBG is not found in what species?

cat

Other than TBG, what else can be involved in the transport of thyroid hormones?

albumin, thyroxine-binding prealbumin

In the absence of TBG, what is the most important carrier of thyroid hormones?

albumin

Concentration of Thyroid hormone that is free in plasma is high/low.

low

What causes an increase in TBG synthesis by the liver?

estrogens

The main form of metabolism of thyroid hormones is what?

removal of iodide molecules

Other than deidination, what other ways are thyroid hormones metabolized?

sulfates and glucuronides formed by liver and kidneys, decarboxylation of thyronine alanine moiety

Deiodinated and conjugated thyronines are eliminated primarily in ____________.

urine

Unmetabolized thyronines are excreted with what?

feces through bile secretion

Degradation of conjugate forms of thyronines in the feces results in what?

production of iodide molecules that are reabsorbed

The mechanism of action of thyroid hormones at the cellular level is based on their ability to do what?

penetrate cell membrane

Thyroid hormones are amino acids and so they are ______________.

lipophilic

Thyroid hormones are the primary determinants of what?

basal metabolism

Thyroid hormones increase oxygen consumption of tissues, what happens as a result?

heat production (calorigenic effect)

What is the calorigenic effect?

increase in body temp

How do thyroid hormones affect carbohydrate metabolism?

increasing intestinal glucose absorption, facilitates glucose movement into fat and muscle, facilitate insulin-mediated glucose uptake

Glycogen formation is facilitated by small amounts of thyroid hormones. True/False

True

Thyroid hormones work in concert with __________________ to ensure normal growth and development.

growth hormone

What is the main effect of thyroid hormones on lipid metabolism?

lipolysis

Thyroid hormones have the tendency to _____________ plasma cholesterol levels.

reduce

The effects of the _____________ nervous system are enhanced by thyroid hormones.

sympathetic

How do thyroid hormones enhance the SNS?

stimulation of B-adrenergic receptors

What is the most important regulator of thyroid activity?

TSH (thyrotropin)

How does TSH act in order to regulate thyroid activity?

initiation of cAMP formation and protein kinase phosphorylation

TSH (Thyrotropin) secretion is regulated by what?

thyroid hormones via negative feedback inhibition of TRH in the hypothalamus and TSH in the pituitary

Inability to secrete adequate amounts of thyroid hormone leads to what?

goiter (enlargement of thyroid gland)

What typically causes goiter?

deficiency of iodine in the diet

What is the usual etiology of primary hypothyroidism in dogs?

lymphocytic thyroiditis

Secondary hypothyroidism in dogs can be a secondary effect of what?

pituitary tumors, radiation, glucocorticoid inhestions

Which type of hypothyroidism can be congenital in dogs as a result of defective TRH or TRH-R?

tertiary

What are the clinical signs of hypothyroidism in dogs?

lethargy, obesity, alopecia, thick skin

What tests are done to confirm primary hypothyroidism?

serum basal T3/T4, serum free T3/T4, endogenous TSH levels

What factors can affect T4 levels in dogs?

age, breed, temp, diurnal rhythm, obesity, malnutrition

What tests results would be expected in a dog with thyroid gland failure?

FT4 and TT4 decrease, increase TSH

What would low FT4 and TSH indicate in a dog?

secondary hypothyroidism

What would low FT4 and high TSH indicate in a dog?

primary hypothyroidism

Hyperthyroidism in cats is caused by what?

hyperplasia of the thyroid

What are the clinical signs of hyperthyroidism in cats?

polyphagia, weight loss, polydipsia, polyuria

Thyroid disease is rare in adult horses. True/False

True

Thyroid disease is not seen in foals. True/False

False (it is)

What tests are performed for equine thyroid function?

serum T3/T4, administer 1mg TRH IV, remeasure T3 in 2 hours and T4 in four hours

In an equine thyroid function test, if there is greater than a 1.5x increase in T3 and T4 this means what?

thyroid is responding to TRH and hypothyroidism can be ruled out

What tests are used for hypothyroid assessment in horses if there is no other plausible explanation for low T3/T4?

FT4 concentration by equilibrium dialysis, TRH stimulation test

In horses, low blood thyroid are most often the body's reaction to the need to do what?

downregulate resting metabolic rate

What are some potential reasons for "low" thyroid hormones in the adult horse?

illness, time of year, time of day, fasting, drugs, exercise, transport, diet

In horses, thyroid levels are highest at birth and decrease throughout their life. True/False

True

Are thyroid concentrations in the horse higher in cold seasons or hot seasons?

cold

Are thyroid concentrations in the horse higher in the morning or the afternoon?

afternoon