Thyroid Gland Flashcards by Karen Fouad

Name the thyroid hormones

Thyroxin or triiodothyronin or T3
Tetraiodothyronin or T4
3l thyrocalcitonin

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List steps of thyroid hormone synthesis

Synthesis & storage of thyroglobulin
Iodide trapoing & oxidation to iodine
Iodination
Coupling
Colloid endocytosis
Cleavage of hormones for release

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The special transported at basolateral surface of follicular cells is……

Sodium Iodide symporter (NIS)

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The products of iodination steps are…..

Monoiodotyrosine
Di-iodotyrosine
Both are inactive

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Most hormones released from thyroid are in form of…….but most of it is converted to……

T4
T3 (more potent)

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T3&T4 are transported bound to……..

Albulmin, pre-albumin, thyroxin-binding globulin (TBG) produced by liver

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GR: T3 is more potent than T4

It is more free
It has higher affinity to thyroid receptors
It is believed that T4 is converted to T3 intracellularly

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Mention tissues whose oxygen consumption is NOT affected by thyroxine

Adult brain, ant pituitary, spleen, lymphoid tissues, retina, testes & uterus

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Describe calorigenic action of thyroid hormones

Inc O2 consumption & BMR
Inc number & size of mitochondria so inc ATP production
On cell membrane in activity of Na/Ka ATPase thus uses energy & produce heat

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Describe effects of thyroid hormones on CHO metabolism

Enhance the intestinal absorption of glucose
Inc rate of glucose uptake by peripheral cells & adipose tissue
Increase late of glycolysis & gluconeogenesis
Potentiate insulin effects & inc secretion
Inc liver glycogenolysis by enhancing actions of epinepherine

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Describe effects of thyroid hormones on fat metabolism

Has powerful lipolytic actions inc FFA in plasma
Inc ocidation of FFAs causing depletion of fat stores & dec in weight
A fall in cholesterol & plasma lipid levels bec of inc excretion of cholesterol in bile & stool, inc LDL receptors in liver

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Describe effects of thyroid hormones on protein metabolism

Normal levels are protein anabolic, high levels cause protein catabolism & may lead to weight loss & muscle weakness (thyrotoxic myopathy)

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Describe effect of thyroid hormone on vitamin metabolism

The general inc in enzyme activity leads to a general inc in body needs for vitamins as it acts as co-factors. They are necessary for coversion of carotednoids to vit A. Deficiency leads to carotenemia, yellowish discoloration of skin.

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Describe effect of thyroid hormones on BMR

Responsible for normal BMR, body weight & energy consumption

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Describe effect of thyroid hormones on growth

A) Physical: stimulate protein synthesis, stimulate GH secretion & potentiate effects
B) Mental: development, growth & function of CNS. Myelination of nerves & development of synapses
C) Sexual: stimulates gonadal function, necessary for normal menstrual cycle & fertility & for spermatogenesis

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Describe effects of thyroid hormones on CVS

Cardiac muscle: +ve inotropic & chronotropic as they inc b-R & sensitise myocardium to CAs, stimulate formation of a-myosin heavy chain which has high myosin ATPase activity, thus stimulate myocardial contractility.
Hemodynamic: inc CO, SV & blood flow to most tissues due to inc metabolism
On BP: inc SBP & PP, dec DBP (due to peripheral VD)

Describe effect of thyroid hormones on following systems:
1. Respiratory
2. GIT
3. Bone marrow

Inc rate & depth of respiration & O2 dissociation by inc 2,3 DPG
Inc appetite, food intake, inc secretion & motility
Thyroid hormones inc erythropoiesis

Describe effect of thyroid hormones on other hormones

Inc rate of secretion of most other endocrine glands, inc insulin & synergistic with CAs. Thyroid hormone effects on cardiac & nervous tissues seem to be mediated by CAs as in hyperthyroidism the symptoms are improved by BBs

Effect of TSH on thyroid gland

Inc:
1. Iodine pump activity
2. Iodination & coupling
3. Secretary activity
4. Release into circulation
5. No & issize of follicles
6. Blood supply of gland

…….inhibit TSH

Dopamine & glucocorticoids

Describe circadian rhythm of TSH

Inc at 9PM, peak at midnight, decline during the day

Adequate iodine intake is…..

150 ug/day

Explain Wolf-Chaikoff effect

Inc iodine dietary intake decreases vascularity & all activities of thyroid gland & consequently:
Decreased organic binding of iodine in thyroid gland, reduce effect of TSH by reducing cAMP response, inhibit proteolysis of thyroglobulin.

List causes of hyperthyroidism

1ry: Grave’s disease, thyroiditis, hypersecreting thyroid adenoma
2ry: pituitary of hypothalamic hypersecretion
Other: exogenous T3/T4, ectopic thyroid tissue.

Describe etiology of Grave’s disease

Autoimmune disease, produces TSI/LATS that mimic TSH & bind its receptor & prementantly switch them on resulting in con thyroid hormone release, no -ve feedback inhibition.

Describe effects of hyperthyroidism on the following: 1. Nervous 2. GIT 3. Metabolic 4. protein metabolism

1. Inc sensitivity of reticular formation to CAs 2. Inc motility & diarrhea 3. Hyperglycemia & dec serum cholesterol 4. Inc protein catabolism with osteoporosis, bone demeralization, hypercalcemia & hypercalcuria. Skeletal muscles are weak & easily fatigued (thyrotoxic myopathy)

GR: Exophalmos occurs with Grave’s disease

Due to swelling of the orbital tissues which pushes eyeballs forward.

List diagnostic tests for hyperthyroidism

1. Measurement of T3,T4(high) 2. Conc of TSH in plasma (low) 3. Conc of TSI by RIA (high in Grave’s) 4. BMR inc (+30 to 60%) 5. Dec plasma cholesterol

List treatment options for hyperthyroidism

1. Thyroidectomy 2. Radioactive iodine destroyes most active thyroid cells 3. Anti-thyroid drugs

List causes of myxedema

1ry: autoimmune gland destruction (Hashimoto’s thyroiditis), surgical removal/irradiation of gland, dietary iodine deficiency 2ry: dec TRH/TSH

GR: Inc weight in hypothyroidism

Dec BMR + accumulation of myxomatous tissue

Mention effects of myxedema on: 1. Skin 2. GIT 3. Nervous 4. Speech 5. Blood lipid 6. Sexual function

1. Dry, scaly, yellowish skin. Generalized non-pitting edema due to myxomatois tissue accumulation 2. Constipation 3. Lethargy, sluggishness, delayed relaxation of tendon reflexes. Dull apathetic look (myxedema madness) 4. Slow speeck, husky voice (diagnosed on phone) 5. Inc blood cholesterol & LDL, HTN & athersclerosis 6. Depresses, amenorrhea in females

List causes of cretinism

1ry: thyroid agenesis, congenital enzyme defects 2ry: maternal iodine def, hypothalamic/pituitary dysfunction Tissue resistance to thyroid hormones

Describe characteristics of dwarfism due to thyroid def

1. Skeletal growth is more affected than soft tissue growth thus there is disproportionate growth 2. Delayed milestones (teeth eruption, sitting, walking) 3. Short obese child with bulging abdomen and umbilocal hernia 4. Enlarged protruding tongue, depressed nose, swollen eyelids 5. Skin manifestations, metal and sexual retardation

Describe effects of myxedema on: 1. Skin 2. GIT 3. Nervous 4. Speech 5. Blood lipid 6. Sexual function

1. Dry, scaly, yellowish skin.