REB 6. Effects of Hypo- and Hyperthyroidism Flashcards
Where is the thyroid gland located?
above the trachea and below the larynx
What shape is the thyroid gland? What can you say about the size of the organ?
- a butterfly or bowtie shape
- it is one of the largest endocrine glands (15 to 20g)
What connects both lobes of the thyroid gland?
the isthmus
What is the isthmus?
it connects both lobes of the thyroid gland
What are the percentages of T4 and T3 secreted by the thyroid gland?
93% of T4 secreted
7% of T3 secreted
*T3 is 10x more potent than T4
What are some characteristics of the response of thyroid hormone?
- response to an increase in TH is detectable only after a delay of several hours
- max. response not evident for days
- duration of response is quite long (this is because TH is not rapidly degraded + response to an increase in secretion of TH continues for days/weeks even after plasma TH concentration has returned to normal)
Is thyroid hormone essential for life?
no, it is not, but it is essential for normal development + physical and mental well being
What is the thyroid response element of DNA?
It is to which the thyroid hormone binds and gene transcription is initiated
- T3 has an increased affinity for DNA than T4
What are some of the general effects of thyroid hormone?
- activates nuclear transcription of large numbers of genes in virtually all cells of the body
- great numbers of proteins - enzymes, structural + transport proteins are synthesized
- net result: generalized increase in functional activity throughout the body
What are the effects of thyroid hormone on metabolic rate and heat production?
- TH increases body’s overall basal metabolic rate (TH regulates body’s rate of oxygen consumption and energy expenditure under resting conditions)
- there is a calorigenic/heat producing effect (the increased metabolic activity results in increased heat production)
What are the effects of thyroid hormone on intermediary metabolism?
- influence synthesis + degradation of fat, carbs and protein
- small and large amounts may induce opposite effects
e. g.- adequate amounts of TH essential for protein synthesis needed for normal bodily growht
- high doses of TH favours protein degradation
What are the effects of thyroid hormone on the nervous system in childhood?
- essential for normal myelination + development of CNS
- hormone deficiency causes mental retardation
What are the effects of thyroid hormone on the nervous system in adults?
- excess causes restlessness + hypersensitivity
- deficiency causes lethargy + blunting of intellect
What are the effects of thyroid hormone on the cardiovascular system?
- increases blood flow + cardiac output
- increase heart rate
- increased heat load generated by TH
(peripheral vasodilation occurs to carry extra heat to body’s surface for elimination)
What are the effects of thyroid hormone on growth?
They are essential for growth because of effects on GH + IGF1
- TH stimulates GH secretion + increases production of IGF-1 by liver
- promotes effects of GH and IGF-1 on synthesis of new structural proteins + skeletal growth
- thyroid deficient children have stunted growth that can be reversed by thyroid replacement therapy [excess TH does not cause excessive growth]
Does excess TH not cause excssive growth?
No, it does not. An excess of GH will lead to excessive growth
What is the most important regulator of TH secretion?
Thyroid Stimulating Hormone (TSH)/thyrotropin released from the anterior pituitary gland
Exposure to cold in newborn infants increase….secretion
TRH
Can emotional reactions affect TRH and TSH?
yes
What are some causes of hypothyroidism?
[1] failure of thyroid gland
[2] autoimmune disease (Hashimoto disease - the body produces antibodies that destroy the thyroid gland)
[3] deficiency of TRH or TSH
[4] inadequate dietary supply of iodine (around 50mg a year)
What are the symptoms of hypothyroidism?
[1] reduced basal metabolic rate
[2] displays poor tolerance to cold (lack of calorigenic effect)
[3] tendency to gain weight (not burning fuels at a normal rate)
[4] easily fatigued/lower energy production
[5] extreme somnolence
[6] slow weak pulse
[7] slow reflexes + slow mental responsiveness
[8] depressed growth of hair
[9] scaliness of skin
[10] development of a frog-like husky voice
[11] in severe cases, oedematous appearance throughout body (myxedema)
What is Cretinism? What are some characteristics of the illness?
Cretinism: when a person has hypothyroidism from birth
- adequate levels of TH is essential for normal growth
Characteristics:
- dwarfism
- mental retardation
- other general symptoms of thryoid deficiency
What are the 2 types of Cretinism? (in other words, the 2 main causes of it)
[1] Congenital Cretinism
- born without a thyroid gland
[2] Endemic Cretinism
- failure of thyroid gland to produce TH (genetic defect)
Why does cretinism only be seen in children only a few weeks after birth? What is the treatment?
neonate with no thyroid gland may have normal appearance and function as it is supplied with TH by mother in utero
- note: need to treat neonate with adequate iodine or thyroxine
- need to be treated within a few weeks to prevent mental growth retardation
What are the diagnostic tests for hypothyroidism?
measure levels of TSH (greatly increased) and measure thyroxine in blood
What is the most common cause (illness) of hyperthyroidism? What are the mechanisms behind it - how does it work?
Graves’ Disease
- autoimmune disease in which Long-Acting Thyroid Stimulator (LATS) target the TSH receptors on thyroid cells
- LATS stimulates secretion and growth of thyroid (similar to TSH)
- LATS not subject to negative feedback inhibition by TH
- thyroid growth + secretion continues unchecked
What are the causes of hyperthyroidism?
[1] excess TRH/TSH
[2] thyroid adenoma
- secretes large quantities of TH
What are the symptoms of hyperthyroidism?
- elevated basal metabolic rate
- resultant increase in heat production (excessive perspiration + poor tolerance of heat)
- increased appetite + food intake (increased metabolic demands)
- body weight falls (burning fuel at abnormally rapid rate)
- net degradation of carbs, fat + protein (loss of skeletal mass)
- heart rate + strength of contraction increases
- effects on CNS (excessive degree of mental alertness)
What is a prominent featureof Graves’ disease?
Exophthalamos
- inflammation + swelling of eye muscles and fat behind the eyes
What is Exopthalamos?
inflammation + swelling of eye muscles and fat behind the eyes
- eyes become dry + irritated
What are diagnostic tests for hyperthyroidism?
- measure concentration of TSH in plasma (very low)
- directly measure thyroxine in plasma
What is a goiter? What can a common cause of this be?
goiter = enlarged thyroid gland (highly visible)
- it occurs when TSH or LATS excessively stimulate thyroid gland
- may accompany hypothyroidism or hyperthyroidism (or may not be present)
In hypothyroidism, why would a goiter develop?
a goiter develops if hypothyroidism results in excessive stimulation of thyroid gland
- thyroid gland failure
- lack of iodine
Steps:
- levels of TH low - little neg. feedback inhibition on anterior pit. + TSH secretion
- elevated TSH acts on thyroid to increase size + number of follicular cells + increase rate of TH secretion
- no TH secreted so TSH promotes hypertrophy + hyperplasia of thyroid [enlarged thyroid gland]
In hypothyroidism, why would a goiter not develop?
no goiter present if thyroid gland is not being adequately stimulated
Why would a goiter arise in hyperthyroidism?
In Graves’ disease, LATS promote the growth of the thyroid
- excessive stimulation of thyroid gland growth
Why would a goiter not arise when hyperthyroidism is present? What are the causes?
Overactivity of thyroid in absence of overstimulation
[1] uncontrolled thyroid tumour
[2] spontaneous secretion of excessive amount of TH inhibits TSH
[3] no stimulus to promote growht of thyroid
