Thyroid, antithyroid Flashcards
How do iodide (I-) levels affect T4 synthesis?
Low I- promotes synthesis, high I- inhibits
T4, T3 biosynthesis: How I- enters colloid space
I- uptaken into follicular cells at basolateral membrane via Na/I symport
I- enters colloid space using Pendrin at apical side in exchange for Cl-
What happens after I- enters colloid space (how does complex end up in follicular cell)
Thyroid Peroxidase (TPO) oxidises I- into I2, then iodinises thyroglobulin. Complex endocytosed into follicular cell by binding to Megalin
How is T4 secreted into bloodstream after endocytosis into follicular cell
Proteolysis to cleave into T4 and T3 by thiol endopeptidases Monocarboxylate transporter (MCT) at basolateral membrane release T3 and T4 into bloodstream
T4 half life
6-8 days (depends on extent of protein binding)
T3 half life
1 day
How are T3 and T4 excreted?
Conjugated with glucuronic acid in the liver, excreted in bile
Wolff-Chaikoff effect and its significance
Autoregulatory phenomenon that occurs during iodine exposure. Excess iodine transported to thyroid gland by NA/I symport
Transient inhibition of TPO and thyroid synthesis.
Can be used for hyperthyroidism treatment, administering large doses of iodine to suppress thyroid gland
Hypothyroidism treatment: Drugs
Levothyroxine (T4), liothyronine (T3) are synthetic preparations of sodium salts of the natural isomers
Main differences between levothyroxine and liothyronine
T4 less active. When given orally, levothyroxine requires 3-5 days onset vs 3h for liothyronine. Levothyroxine requires 3-4 months to reach steady state vs 2 wks for liothyronine. Hence, levo is used for chronic thyroid replacement while lio is used when rapid tx is needed (myxedema coma)
Levothyroxine: ADR
Long term use associated with increased bone resorption and reduced bone density especially in post-menopausal women
Need to monitor for persistent TSH elevation if dose is inadequate
Risk of MI, angina in elderly, need to administer small doses
Liothyronine: ADR
Easy to OD, risk of cardiac carrythmia and MI
Levothyroxine: DDI, food
Taken 30-45min before food on empty stomach. Dose needs to increase with estrogen replacement treatment
In what special groups does levothyroxine dose need to be lowered?
Elderly, IDH patients
In what special group does levothyroxine dose need to be increased?
Pregnant patients w hypothyroidism
Hyperthyroidism: Drugs
Thioamides
High doses of I-
Radioactive iodine
Thioamides : Class, names, moa, important adr
Hyperthyroidism (anti-thyroid)
Carbimazole, propylthiouracil
Inhibit TPO, inhibit oxidation and iodination. Propylthiouracil further inhibits deiodination of T4 to T3.
Agranulocytosis - watch out for fever within first 3 months
Mild rash
Cholestatic jaundice
Carbimazole vs propylthiouracil main differences
Propylthiouracil should be reserved for those who cannot tolerate thioamides, risk of serious liver injury
1st trimester: propylthiouracil (may induce goitre during organogenesis)
2nd-3rd trimester: carbimazole (avoid hepatotoxicity)
Iodide: Class, names, moa, adr, contraindications
Hyperthyroidism drugs
Lugol’s solution, KI
*Wolff-Chaikoff effect, inhibit iodine uptake, coupling and release of T4/T3
Iatrogenic thyrotoxicosis - iodine escape
PREGNANCY, BREASTFEEDING -may cause fetal goiter
Chronic intoxication -metallic taste
Why is iodide advantageous for thyroidectomy pre-op?
Reduces thyroid synthesis, thyroid size and vascularity
Why is iodide advantageous for post-radioactive iodine therapy?
Used to block uptake of radioactive iodine by thyroid, reducing thyroid cancer risk, avoid thyrotoxic crisis
Radioactive iodine: Class, names, moa, adr, DDI, contraindication
Hyperthyroidism drugs
131, 123I
Taken up into follicular cells via Na/I symport, release destructive b-particles, spares surrounding tissue, causing pyknosis and necrosis of gland
Cannot use with thioamides, affects efficacy
High incidence of delayed hypothyroidism
Small chance of stomach, kidney, breast cancer (express Na/I symport)
PREGNANT WOMEN
