Thyroid Hormones

How many lobes does the thyroid gland consist of?

2

Where are the lobes of the thyroid gland located?

each side of the trachea below the larynx

What are the major cell type in the thyroid gland?

follicular cells

What are the follicles filled with?

colloid

What amino acid is thyroid hormone derived from?

tyrosine

What does thyroid hormone incorporate?

iodine

What is the only source of iodine?

diet

What is the precursor to thyroid hormones?

thyroglobulin

Where is thyroglobulin stored?

colloid of follicle

What are the thyroid hormones?

T4 (thyroxine) and T3 (triiodothyronine)

What proportion of total hormone released from thyroid gland is T3?

10%

What proportion of total hormone released from the thyroid gland is T4?

90%

What is the activity (receptor affinity) of T3?

more active (5X)

What is the activity (receptor affinity) of T4?

less active

What is the half-life of T3?

1 day

What is the half-life of T4?

6 dyas

Why does T4 have a longer half-life than T3?

T4 bound to circulatory proetin more tightly

Are most of the physiological effects of thyroid hormones due to T3 or T4?

T3

What are the stages of he hypothalamo-pituitary-thyroid axis?

1. hypothalamus release TRH

2. TRH go ant pit, bind to thyrotrophs, stimulate TSH production

3. TSH released into circulation

What is TSH released in response to?

TRH

TRH

thyrotropin releasing hormone

TSH

thyroid stimulating hormone

What does TSH bind to?

receptor on thyroid follicular cell

What does TSH do?

stimulate functions of thyroid gland

What are the functions of the thyroid gland?

• ^ iodine uptake

• ^ protein synthesis

• ^ re-uptake of colloidal thyroglobulin

What doe increased iodine uptake, protein synthesis and re-uptake of colloidal thyroglobulin cause?

increased synthesis and release of T3 and T4

What happens to TRH and TSH when T4/T3 is high?

they’re suppressed

Stages of action of thyroid hormone on target cells

1. T3/T4 enter target cell

2. T4 → T3

3. T3 bind to THR (in nucleus)

4. THR bind to promotor & activate gene transcription

THR

thyroid hormone receptor

What physiological actions does TH effect?

basal metabolic rate, cardiovascular and growth

How does TH affect the cardiovascular system?

long-term sensitivity of cardiac cells regulated by plasma cells of TH

What is TH critical for (relating to growth)?

development of CNS

What does lack of T3/T4 in late foetal early neonatal period cause?

irreversible failure of CNS development

Why does lack of T3/T4 in the late foetal/early neonatal period cause irreversible failure of CNS development?

reduced number of neurons and myelination

Basal Metabolic Rate (BMR)

rate at which the body uses energy to maintain vital functions whilst at rest

hyperthyroidism

excess thyroid hormone

What is basal metabolic rate measured by?

increased heat production and oxygen consumption

How does TH impact increased metabolic rate?

more glucose made to meet elevated metabolic demand

How does hyperthyroidism impact the cardiovascular system?

^HR

How does hyperthyroidism impact body weight?

weight loss (with ^appetite)

How does hyperthyroidism impact anatomy?

enlarged thyroid gland (goiter)

What are other symptoms of hyperthyroidism?

hyperactivity, nervousness, irritability, poor coat

hypothyroidism

thyroid hormone deficiency

How does hypothyroidism impact the cardiovascular system?

decreased HR

How does hypothyroidism impact body weight?

weight gain (no corresponding increase in appetite)

How does hypothyroidism impact anatomy?

enlarged thyroid gland (goiter) depending on cause

What are other symptoms of hypothyroidism?

cold intolerance, fatigue

In what species is hypothyroidism recognised?

dogs (predisposition is some breeds)

What can hypothyroidism be caused by/associated with?

• iodine deficiency

• gland enlargement

What causes the thyroid gland to enlarge due to hypothyroidism?

continued stimulation of thyroglobulin by TSH due to lack of feedback control of TSH

What cells does TSG bind to?

follicular cells

What does the anatomy of parathyroid glands vary with?

species

What is the general anatomy of the parathyroid glands?

2 paired, at least one pair associated with thyroid gland

What do parathyroid glands consist of?

• densely packed chief cell

• rich capillary network

What do chief cells in the parathyroid gland synthesise and secrete?

parathyroid hormone (PTH)

What cells in the parathyroid gland stimulate and secrete parathyroid hormone (PTH)?

chief cells

What is PTH crucial for?

calcium homeostasis

What is PTH released in response to?

low calcium levels in circulation

Why is the parathyroid gland clinically relevant?

can be inadvertently removed during thyroid surgery