Iodide is taken up by the thyroid and oxidized to a more reactive form via thyroid peroxidase.  This same enzyme then catalyzes the addition of the tyrosine residues iodide to thyroglubulin, forming mono and diiodinated tyrosine.  These residues are then coupled, forming either T3 (MIT+ DIT) or T4 (DIT+ DIT).

These will be stored in the colloid until needed, at which point TSH will stimulate the endocytosis of the thyroglobulin, which is then proteolyzed to release the T3 and T4 molecules.  This will be extensively bound to TBG (thyroid binding globulin) in the blood.

Levothyroxine sodium (T4) is the preparation of choice for thyroid hormone replacement therapy.

It is prefered over Liothyronine sodium (T3) because liothyronine is cardio-toxic.

Iodide, lithium, amiodarone can all cause hypothyroidism.

However, the most common cause of hypothyroidism is the autoimmune disorder 'Hashimoto's thyroiditis"

The thioamides are also known as "anti-thyroid drugs".  They include Propylthiouracil and methimazole.

These drugs inhibit thyroid peroxidase!

Methimazole is preferred because it is more potent and longer lasting.  However, propylthiouracil is preferred in pregnancy and has peripheral inhibition of T4-T3 conversion..

Methimazole (and propylthiouracil) inhibit thyroid peroxidase.  They therefore inhibit the formation of T3 and T4 by blocking oxidation, iodination, and coupling steps of thyroid hormone synthesis.

They do NOT affect iodide uptake, only its use once the thyroid has taken it up
It also does NOT immediately affect its release, which is why its effects are not fully felt for a few weeks.

Propylthiouracil also inhibits peripheral conversion of T4 to T3.

The thioamide Propylthiouracil inhibits conversion of T4 to T3 in the peripheral tissues.

Beta-blockers also have this effect!

The thioamides are propylthiouracil and methimazole.  They are used in cases of hyperthyroidism such as graves disease, thyroid storm etc.  They inhibit thyroid peroxidase.

They can cause lupoid reactions, hypothyroidism, and rarely: agranulocytosis (a serious condition that necessitates immediate withdrawal of the thioamide and treatment with filgrastim/ G-CSF stimulating factor)

Iodides, ex: potassium iodide, block thyroglobulin proteolysis, and therefore release of T3 and T4.  They also promote the Wolff-Chaikoff effect, decreasing synthesis by inhibiting organification (addition of iodide to thyroglobulin).

Also important as a pre-surgery drug because it reduces the vascularity of the thyroid, helping to minimize risks involved in the surgery!

Thyroid storm is a medical emergency.  In emergency situations, the iodides like iodine or potassium iodide are preferred because they have a faster onset of action.  Recall that potassium iodide inhibits the release of thyroid hormone! (as well as synthesis)

Iodides also decrease the size and vascularity of the thyroid, allowing for an easier, less complicated surgery :)

Also give beta blockers (symptomatic treatment), and propylthiouracil (blocks peripheral conversion)

Lugol's Solution is a mixture of iodine and potassium iodide.  It is given before surgical resection of the gland.  Recall that iodides decrease the size and vascularity of the thyroid!

(contraindicated in pregnancy)

131-I

It is administered orally, is taken up and concentrated by follicles, and destroys the gland.

Do not use in pregnancy or in mothers with small children!

recall: 123-I is used for thyroid scanning and diagnosis.

Beta blockers can reduce the symptoms (mainly cardiovascular) of hyperthyroidism while the other drugs like thioamides, iodides, radioactive iodine can target the disease itself.

Also recall that beta-blockers can decrease peripheral conversion of T4 to T3

Thioamides do not affect the release of thyroid hormone, only the synthesis (by blocking thyroid peroxidase).  Their effect will therefore only be felt once the pool of thyroid hormone in the follicles has been depleted, which will take 3-4 weeks.

This is why iodides, which block release, and beta blockers must be used in thyroid emergencies (i.e. thyroid storm)