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29 Cards in this Set
- Front
- Back
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Thyroid hormones
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-100% of T4 is made in the thyroid
-99.9% of T4 is protein bound -99.7% of T3 is protein bound -Fate of secreted T4: T3 (33%), rT3 (40%), T4 (27%) -80% of circulating T3 is made in periphery -T3 3-4x more potent than T4 |
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Thyroid Hormone Binding Proteins
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-thyroid/thyroxine binding globulin (TBG)
-thyroid binding prealbumin -thyroid binding albumin |
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Thyroid Hormone serum level variables
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1. TH bindg protein concentration
-inc TBG (estrogens, acute hepatitis) -dec TBG (glucocorticoids, nephrotic syndrome) 2. Drug/chemical binding site competition -phenytoin and salicylates 3. Peripheral conversion rates (some drugs can affect the conversion rate from T4 to T3 like propanolol dec the rate of conversion) 4. synthesis and secretion from thyroid |
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Thyroid hormone- some specific effects
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1. Carbohydrate, lipid and protein metabolism
2. Oxygen consumption and heat production 3. Bone (osteoblast/osteoclast activation) 4. Brain (axonal growth and development) 5. Pituitary (growth hormone stimulation) 6. Heart (increases cardiac output and decreases vascular resistance) 7. Gastrointestinal tract (stimulates motility) 8. Sexual organs (maturation) |
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Hypothalamic-Pituitary Axis
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TRH made in hypothalamus --> acts on ant pituitary and stimulates release of TSH--> acts on thyroid --> increases the size and number of the follicular cells, promotes the progression of the thyroglobulin molecule, and increases iodide uptake by the thyroid --> thyroid makes more T3 and T4 and it goes to the peripheral tissues
-negative feedback loop -any minor changes that the hypothalamus and pituitary sense they cause a major change |
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Hypothyroidism
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-serum level of thyroid hormone is insufficient to provide for cellular metabolic needs
-hashimotos thyroiditis- AI disorder -Radioactive iodine tx -surgical removal of thyroid -F>M -30-60 yrs old |
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common signs and symptoms of hypothyroidism
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1. enlarged thyroid
2. fatigue 3. constipation 4. bradycardia 5. impaired cognition/memory 6. intolerance to cold 7. dry skin 8. excessive menstrual bleeding |
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Primary hypothyroidism
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-Low T3 and T4 with high TSH
-thyroid itself is not working therefore not making a lot of T3 and T4 but the negative feedback causes the hypothal to produce TSH |
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Secondary hypothyroidism
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-Low T3 and T4 with low TSH
-problem in the pituitary or hypothalamus (NOT the thyroid) -nothing is stimulating the thyroid so there is low T3 and T4 so it feeds back by the pituitary is not working so not matter how much you stimulate it you wont make it work |
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Hyperthyroidism
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-excessive thyroid hormone in the circulation causing increased metabolic activity
-Graves disease -Toxic multinodular goiter -single toxic nodule -F >>M -onset with puberty/pregnancy/menopause/ stress |
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common ssx of hyperthyroidism
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1. wt loss
2. insomnia 3. amenorrhea 4. tachycardia 5. nervousness 6. fine tremor 7. heat intolerance 8. warm, moist skin -primary (low TSH, high T3 and T4), secondary (high TSH, high T3 and T4), tertiary |
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indications for thyroid function tests
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-screen high risk groups
-newly diagnosed hyperlipidemia -women >50 -newborns -positive FH -elderly -AI disorders -endocrine disorders -pts with suspected thyroid disorders -monitor pts being treated for thyroid disease |
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Thyroid Stimulating Hormone (TSH)
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-measures TSH concentration in the serum
-most sensitive assay for detection of primary thyroid disease and for monitoring replacement and suppression hormone therapy -pt preparation: no significant dietary or time restrictions -sample: 7 ml venous blood in red top/SST tube |
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TSH clinical implications
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-inc TSH --> primary hypothyroidism, surgical removal of thyroid
-dec TSH --> primary hyperthyroidism, factitious hyperthyroidism (taking T3 and T4 to lose weight) -N/dec TSH --> secondary or tertiary hypothyroidism |
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Total thyroxine (TT4)
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-measures total T4 concentration in the serum (including bound and free T4)
-serum level depends on protein status, pt health and rate of conversion -pt prep: no dietary or time restrictions -sample: 5 ml venous blood in red top/SST tube (serum) -by itself not a good screening test |
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clinical implications of inc TT4
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-hyperthyroidism
-ince thyroid binding proteins: preg, OCs, congenital anomaly, acute hepatitis -factitious hyperthyroidism |
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clinical implications of dec TT4
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-hypothyroidism
-protein binding site competition by drugs -dec thyroid binding proteins: malnutrition, congenital deficiency, nephritic syndrome, liver failure, chronic illness, glucocorticoids -anti-thyroid drugs -iodine insufficiency -euthyroid sick syndrome |
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Reverse Tri-iodothyronine (rT3)
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-metabolically inactive form of T3
-order to resolve conflicting lab data -impaired conversion of T4-->T3 -elevated in "euthyroid sick syndrome) acute illness (infarct, fever, trauma, burns, surgery) chronic illness (DM, cancer, renal failure) |
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Total Tri-Iodothyronine (TT3)
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-measures total T3 in the serum (bound + free T3)
-pt prep: no dietary or time restrictions sample: 5 ml venous bl in red top/SSt tube -no advantage over TT4 |
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T3 Resin uptake
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-estimates the number of available thyroid hormone protein binding sites
-pt prep: no dietary or time restrictions -sample: 5 ml venous blood in red top/SST tube |
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low T3 resin uptake clinical implications
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-hypothyroidism
-pregnancy -genetic TBG excess -acute hepatitis |
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high T3 resin uptake clinical implications
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-hyperthyroidism
-malnutrition -genetic TBG deficiency -chronic liver disease -nephrotic syndrome |
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Free thyroxine index FF4I
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-RR 1-4
-indirect measurement of free thyroid hormone FT4I = TT4 x pt T3RU (as a decimal) |
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clinical implications of free thyroxine index
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-inc FT4I : hyperthyroidism
-dec FT4I: hypothyroidism, severe nonthyroidal illnes |
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Free T4 and Free T3
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-measures metabolically active forms of thyroid hormone in serum
-unaffected by serum protein concentrations -pt prep: no dietary or time restrictions -sample: % ml venous blood in red top/SST -sensitivity and specificity > FT4I -expensive |
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clinical implications of Free T4 and Free T3
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-inc:
1. hyperthyroidism 2. factitious hyperthryoidism 3. acute thyroiditis 4. ectopic thyroid tissue -dec: 1. hypothyroidism 2. iodine insufficiency |
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Thyroid stimulating ABs (TSI)
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-also called TSH receptor antibodies
-90% of patients with Graves disease are positive -not typically required to make diagnosis |
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Thyroid microsomal ABs (TMAb)
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-also called thyroid peroxidase antibody (TPO)
->90% of patients with autoimmune thyroid disease are positive |
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Anti-thyroglobulin ABs (TgAb)
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-sensitivity for hypothyroidism 50%
-useful in patients with thyroid cancer and thyroglobulin testing (interfering factor) |
