Thyroid Flashcards
Where thyroglobulin is stored
colloid
2 important TSH-inducible transcription factors
PAX-8, TTF-1
alpha subunits that TSH GPCR is coupled to
Galpha-s (normal) and Galpha-q (only at high TSH levels)
What is used to uptake iodide into follicular cells
Sodium-iodide symporter (NIS): 2Na for every 1 iodide
Enzyme that oxidizes iodide using H2O2 and is localized on the apical side of the thyrocyte (near the colloid) and also allows coupling of 2 tyrosine rings (oxidative condensation)
Thyroid peroxidase (TPO)
The predominant thyroid hormone in circulation
T4
The active form of thyroid hormone that has much higher affinity for thyroid receptor
T3
Enzymes that convert T4 to T3 within target cells
Deiodinases
Synthetic T4
levothyroxine
Synthetic T3
liothyronine
2 thionamides used to treat hyperthyroidism
methimazole, propylthiouracil
Target of thionamides
thyoid peroxidase, thus inhibiting iodination of thyroglobulin and coupling of MIT and DIT to generate T3 and T4
Additional mechanism of propylthiouracil besides targeting TPO
Inhibits D1 and D2, inhibiting peripheral conversion of T4 to T3
MOA of radioactive iodine
Uptake by NIS and concentrated in the colloid where it emits beta particles that destroy the follicle
Ionic inhibitor used for hyperthyroidism that competitively inhibits NIS
Perchlorate
Primary function of thyroid hormones
Maintain metabolism in the body
Function of hypothalamus in thyroid regulation
Releases thyrotropin-releasing hormone (TRH)
Function of the pituitary in thyroid regulation
releases thyrotropin, also known as thyroid-stimulating hormone (TSH)
Normal TSH level
0.5 - 2.5 mcgU/mL
Normal T4 levels
0.7-1.9 Ngo/mL
autoimmune hypothyroidism
Hashimoto’s disease
The precursor for thyroid hormones
Thyroglobulin (TG)
Normal starting dose of levothyroxine for hypothyroidism
1-1.6 mcg/kg/day (use IBW for obesity)
Starting levothyroxine dose for elderly and patients with CVD
25-50 mcg
Reason why synthetic T4 (levothyroxine) is used over synthetic T3 (liothyronine)
half-life of T4 is 6-7 days, whereas T3 is only 2 days
How often TSH/free T4 should be monitored in a pt with uncontrolled hypothyroidism
every 6-8 weeks
How much should a levothyroxine dose be titrated by
10-20%
How often TSH/free T4 should be checked in a patient with controlled hypothyroidism
annually or if symptoms
Levothyroxine dose conversion from PO to IV
Give 1/2 of normal PO dose
Dose of levothyroxine if a patient has subclinical hypothyroidism (TSH 5-10 mcgU/mL)
25-50 mcg/day
Very advanced hypothyroidism with very high TSH linked to 60-70% mortality
myxedema coma
Treatment of myxedema coma
IV levothyroxine and corticosteriods
Autoimmune hyperthyroidism
Grave’s disease
3 drugs that can cause hypothyroidism
- lithium
- amiodarone
- interferon-alpha
BB that can be used for symptomatic relief of tachycardia associated with hyperthyroidism and also inhibits peripheral conversion of T4 to T3
Propranolol
Thionamide that is preferred in pregnancy
Propylthiouracil
Labs needed for thionamide monitoring
- TSH/fT4 every 4-6 weeks
- LFTs
- CBC (if reason such as cough or fever)
3 parts of treatment for thyroid storm (severe thyrotoxicosis)
- IV BB and iodide
- Methimazole or PTU
- hydrocortisone
