Thyroid Hormones Flashcards
Thyroid Hormone Synthesis
- Iodide Trapping (Through Na/I Symporter)
- Oxidation of Iodide to Iodine (by Thyroid Peroxidase “TPO”)
- Iodination of Tyrosine (Iodine Organification
- Coupling (Diiodotyrosine and Monoiodotyrosine)
Thyroid Hormones Plasma Proteins
Thyroxine-bing Globulin (TBG)
Thyroxine-binding Prealbumin (TBPA)
T4 has higher binding affinity than T3.
Metabolism of Thyroid Hormones
Sequential Deiodination.
Amiodarone, Propranolol, Corticosteroids and PTU inhibit peripheral conversion of T4 to T3
Thyroid Hormones t1/2
T3—1 day only.
T4—5-7 days.
*STEADY STATE: 6 half lives.
TSH Test
It is the most sensitive test to evaluate thyroid function.
It must be re-evaluated every 4-6 weeks, until normal, then 4-6 months then yearly.
Medications and Thyroid Function Test
- Oral Contraceptives increase TBG binding capacity.
- Aspirin, Androgens and Corticosteroids decrease TBG binding capacity and displacement of T4.
- Anti-Epileptics increase T4 clearance by enzyme induction.
- Bromocriptine decreases TSH secretion.
- Metoclopramide and Domperidone increase TSH secretion.
Hashimoto’s Thyroditis
Hypothyrodism
Drug-induced Hypothyrodism
Lithium
Amiodarone
Sunitinib
Myxedema Coma “Crisis” Management
L-Thyroxine 400-500mcg IV (<55 years and no CVD), 50-100 mg Hydrocortisone every 6 hours.
Levothyroxine
Drug of choice for Hypothyrodism.
Start with 25-50% of the dose. (Titrate)
Absorption is optimal on an empty stomach, 60 mins before breakfast or 3-4 hours after meals. Calcium and aluminum decrease its’ absorption, as well as coffee and fatty meals.
For T4 analysis—8-10 hours after the daily dose.
Graves’ Disease
Hyperthyrodism
It is an autoimmune disease; autoantibodies mimic TSH effects, they active Thyroid Gland TSH receptors.
PTU can be used from the first day of life, while Methimazole can be used after the age of 2 years old.
They can also be used safely in lactating mothers.
