How is hypothyroidism managed? 

What is the exception to this? 

Levothyroxine (thyroid hormone T4)

• Levothyroxine → liothyronine (T3) 
• Liothyronine has a shorter half-life so it is not used (7 days vs 1 day) so you can use levothyroxine once 

The exception is hypothyroid disease caused by low iodine 

• Can use either levothyroxine or liothyronine in cases of myxedema coma caused by a hypothyroid state.  
• However, be careful because elevated levels of either can increase mortality 

• Be careful when giving it to paitents with cardiac disease particularly when they have arrhythmias 

• Amioderone is used to treat cardiac arrythmias 
• It blocks the action of deiodinase that is required from converting T4 → T3.  So this causes low levels of T3 

Thiocyanate (SCN-) 

Perchlorate (ClO4-)

NIS transporter is the co-transporter at the basolateral surface of the follicular cell that transports Iodine and Na together 

Thioamides block:

• Iodination of tyrosine residues on thyroglobulin 
• Coupling of the iodinated tyrosines on the thyroglobulin molecules

Propylthiouriacil (PTU)

Methimazole (MMI)

Thioamide drug

Inhibits thyroid hormone synthesis 

Thioamide

Inhibits thyroid hormone synthesis

• Proteolysis of thyroglobulin is required for the relase of TH 
• Tg degradation begins with colloid endocytosis and colloid fusion with lysosomes 
• Further degradation of of Tg → cuases degradation of DIT and MIT and allows for iodide reclycing 
• Lastly there is an intra-thyroidal converstion of T4 to T3 done by type-I and type-II deiodinases 

• Inhibits coupling of iodinated tyrosine 
• Inhibits iodination of tyrosine residues
• Inhibits intra-thyroid and extra-thyroid converstion of T4 to T4 via type-I deiodinase 

Short half-lives 

Necessitate multiple doseases a day at the beginning of therapy and then allows for tapering of regimen to once a day 

• Liver failure:  Risk is greater for PTU than for MMI.  Acute, severe, liver failure has lead to death and causes for a black box warning 

• High doses of iodide salts reduce TH synthesis and release 

Iodide salts 

Examples

Use

Potassium iodide (saturated solution)

• Reduce synthesis and release of TH 
• Used to treat thyroid storm 
• Decrease thyroid vascularity → used to prepare for thyroidectomy 

• Iodide is concentrated in the colloid
• Used to target follicular epithlium 
• Also used to scan for thyroid caricnoma metastases 

• Propranolol:  Used to decrease cardiac response to excess TH 
• Hydrocortisone:  Used to treat shock 
• KI (potassium-iodide):  Reduce synthesis + release of TH
• PTU:  Blocks synthesis of TH