Thyroid Disorders Flashcards
1
Q
T3 vs T4
A
- thyroxine (T4) = 90% of hormone secreted rom thyroid
- T4 converted to T3 in peripheral tissues
- Thyroid hormone maintains energy, homeostasis, and regulates metabolism
- target organs include GI, heart, brain, bone liver, skin
- thyroid receptors found in virtually all cells
- T3 is released way less than T4 but is much more metabolically active
- T4 acts as a prohormone for T3
- T4 has a 100 fold lower affinity than T3 for thyroid receptor
- thyroid receptors are found in the nucleus
2
Q
free vs bound thyroid hormone
A
- tightly but reversibly bind to plasma proteins (70% bound to thyroid binding globulin (TBG))
- plasma concentrations of free T4 and T3 reflect amount of hormone actively exerting effect on tissues
- clinically, freeT4 (FT4) is used more than totalT4 (TT4)
3
Q
iodine’s role in thyroid hormone
A
- essential for synthesis of thyroid hormones - obtained from our diet
- dietary iodine = from table salt, ocean fish, seaweed, dairy, grains, kale, spinach
4
Q
factors that increase TBG
A
- estrogens (exogenous or due to pregnancy)
- hypothyroidism
- acute hepatitis
- cirrhosis
- When levels of TBG are increased, more free thyroid hormone is bound so less available to exert effects
- ex) pregnant women needs to have higher dose of T4 or T3
5
Q
factors that decrease TBG
A
- androgens
- cirrhoses
- menopause
- glucocorticoids
- nephrotic syndrome
- cushing’s syndrome
6
Q
labs to assess thyroid hormone
A
- TFTs (thyroid function tests)
- TSH: normal range = 0.4-4.0 –> elevated in hypothyroidism, decreased in hyperthyroidism
- ***TSH ALONE IS THE GOLD STANDARD FOR ASSESSING THYROID CONDITIONS
- free T4: direct measurement of unbound thyroxine
- helpful to look at TSH and T4 together
- start by looking at TSH alone
- free T4 should be obtained to quantify the degree of hyper or hypothyroidism
- T3: serum level useful in dx of autoimmune hyperthyroidism
- thyroid antibodies (TPOAb, TgAb, TSI, TRAb, TBII): useful in diagnosis of autoimmune thyroid dz –> TPOAb and TgAb bositive in Hashimoto’s and Grave’s dz, TBII, TSI, TRAb only found in Grave’s dz
- thyroid scintiscan: not ordered first
- reverse T3: not very helpful unless you suspect euthyroid sick syndrom (common in elderly with sepsis)
7
Q
interpreting labs
A
- serum TSH and FT4 normal –> no further testing
- serum TSH high: low free T4 = primary hypothy, high free T4 = secondary hyperthy (very rare)
- Serum TSH low: low free T4 = secondary hypothy (rare), high free T4 = primary hyperthy
- thyroid uptake and scan: increased uptake = hyperthy, decreased uptake = hypothy
8
Q
hypothyroidism
A
- hypometabolic state from insufficient thyroid hormone
- primary = high TSH with low T4 (thyroid gland problem)
- secondary = low TSH with low T4 (pituitary or hypothalamic - less common)
9
Q
hypothyroidism etiology
A
- most common = dietary iodine deficiency (developing countries)
- autoimmune = destruction of thyroid (hashimoto’s) –> anti-thyroid peroxidase Abs, anti-thyroglobulin Abs
- iatrogenic = results from hyperthyroid or thyroid cancer treatment (RAI, thyroidectomy)
- drug induced = lithium, amiodarone, thalidomide
- primary hypothyroidism = 95% cases
- affects women 4x more than men
- most common in developed countries = Hashimotos
10
Q
Clinical features of hypothyroidism
A
- onset is insidious
- fatigue, lethargy, increased sleep, impaired mental fn, depression, cold intolerance, weakness, myalgias, weight gain, hair loss, skin and nail changes (dry, brittle, scaling), constipation, hoarseness, dysphagia, edema (most is facial and periorbital), menstrual irregularities (increased risk miscarriage, infertility)
- goiter, bradycardia, decreased tendon reflexes, macroglossia
11
Q
Hashimoto’s thyroiditis
A
- chronic lymphocytic thyroiditis
- common cause hypothyroidism (most commonly affects women age 30-60, family hx present)
- high titers autoantibodies (TOPAb and/or TgAb)
- goiter is common finding
- 8x more common in women!
12
Q
Tx of Hypothyroidism
A
- thyroid hormone replacement: levothyroxin (synthroid, levoxyl, levothyroid, tirosint) –> dosing = 1.7 mcg/kg po qd (best taken on empty stomach
- conservative initial tx; incremental increase to slowly achieve restoration of normal metabolism (used in children, elderly, cardiac pts
- full clinical response takes several months, repeat labs every 4-6 weeks
- typically start adults at 25-50 mcg dose (unless they need full thyroid replacement)
- other tx = dessicated pig thyroid gland - includes T4 and T3
- LEVOTHYROXINE is first line due to data, safety
13
Q
complications of hypothyroidism: myxedema coma
A
- severe hypothyroidism, often fatal
- more typically in edlerly with long term untreated hypothyroidism
- usually preceded by event (PNA, peritonitis, MI, CVA, trauma)
- EMERGENCY - high mortality rate
- rarely seen due to use of TSH assays and early detection
14
Q
clinical features of myxedema coma
A
- myxedema = general skin and soft tissue swelling (abnormal deposits of mucin –> proteins produced by epithelial tissues, maintains mucosal barriers)
- scaling, dry, cold; appear “doughy”, yellow-orange discoloration; periorbital edema, nonpitting edema throughout
- sinus bradycardia, hypotension, hypoventilation, pericardial effusion, pleural effusion, ascites
- decreased mental status, slowed speech, ataxia
15
Q
hyperthyroidism
A
- overproduction of thyroid hormone
- MCC is autoimmune –> Grave’s disease
- other causes = toxic nodular goiter (single or multinodular), thyroiditis (subacute, painles, postpartum), TSH producing adenoma (rare)
