Welcome to pharmacology by Lecturio. I’m Dr. Pravin J. Shukle
and today we’re going to cover the thyroid.
Now, we use many drugs in thyroid disease.
We use drugs to treat hypothyroidism and we use drugs to treat hyperthyroidism.
Focusing on low thyroid states, we often use T4 analogs or T3 analogs.
The common brands names of levothyroxine include the following.
It’s a huge list, I’m not gonna go into all of them,
bottom line is that the most common ones that we use are Eltroxin and Synthroid.
In terms of these synthetic versions of the T4 molecule,
they are available in oral, intravenous and intramuscular formulations.
They are the most commonly prescribed medication in the Unites States.
We use it obviously for the treatment of low thyroid
and we’ve also used it in the treatment of goiter.
We also lower TSH with these medications and that’s how they function in the treatment of goiter.
Here’s an example or a picture of goiter that was released in the late 1800s
and that can show the massive enlargement of the thyroid gland that can occur over years.
We also get myxedema from severe hypothyroidism and myxedema
can progress to the point of having myxedema coma.
This is a patient from the 1940s who has myxedema.
You can see that he has the periorbital edema, the sunken expression, very low energy
and he may even have a little bit of abdominal swelling with ascites.
Of course these patients will also have pretibial myxedema with thickening of the tissues
on the anterior aspect of the shin that is not pitting
and that’s what distinguishes it from say cardiogenic peripheral edema.
These medications are taken on an empty stomach.
Generally speaking, I recommend that patients take it 30 minutes before a meal.
Bedtime dosing is also as affective as morning dosing as long as it’s taken away from eating.
In terms of pregnancy, we sometimes have a requirement to increase our dosages.
Generally speaking, we go from seven does a week to nine doses a week in treating pregnant patients
and we recheck the blood work every six weeks.
Now one of the most common questions on internal medicine exams is,
how do you treat a pregnant patient with the thyroid disease?
So this is something that’s going to be important at that level.
In terms of breastfeeding, there’s very small amount of these drugs secreted in breast milk
so there’s no real associated difficulty or problem in patients
who have taken these medications and were breastfeeding, there’s no effects on the fetus or child.
In terms of lactation, you do need adequate thyroid for effective lactation
so when patient’s thyroid levels tend to drop with say inadequate dosing, their breastfeeding also drops off.
In patients with ischemic heart disease who have been diagnosed
with low thyroid initiated at half the normal dose
because you don’t wanna induce any kind of tachycardia that could put risk to the myocardium.
Side effects with this medication can include allergy so one has to be aware of that.
Allergy can sometimes occur with quite severe consequences like shortness of breath, swelling of the face,
and sometimes even swelling of the tongue so be aware in some patients
who are having these allergic responses that it could be the thyroid medication.
We also worry about patients overdosing on these medications.
They tend to show up with clinical signs of hyperthyroidism
such as palpitations, tachycardia and perhaps even arrhythmia.
We also worry about abdominal pain and diarrhea in these patients
and they become quite agitated with insomnia and tremulousness.
Sometimes with prolonged usage or prolonged over usage, people will lose weight
and they’ll have an increase in their appetite.
There are many patients who abuse the thyroid medication as a weight loss attempt
and so be aware that sometimes your patients will actually abuse their thyroid medications for other gain.
People who have overdosed with these medications also complain of heat intolerance
and they feel hot to the touch. They will often have fever, they will often have low blood sugars,
they may exacerbate heart failure or develop heart failure and they could end up in a comma.
One of the more complicated effects of thyroid overdose is an unrecognized adrenal insufficiency
because it masks the symptoms of adrenal insufficiency.
Remember that massive overdoses may cause excessive tachycardia and arrhythmia
and you have to treat that with beta blockers.
Some of the drug interactions associated with L-thyroxine include ketamine
which can cause hypertension and tachycardia, tricyclic antidepressants
can cause an increase toxicity of the thyroid and lithium can affect the iodine metabolism
and cause increased thyroid levels.
In terms of reduced absorption, a lot of times when you take antacids,
they will reduce the absorption of these agent so you could have reduced levels of thyroid
with the same drug in the presence of aluminum containing antacids and magnesium containing antacids.
Similarly, drugs like sucralfate, cholestyramine, colestipol
which are binding resins in the gut, can bind the thyroid as well and cause low drug levels.
Simethicone is an anti-gas medication that can interfere with the absorption and finally bran,
just the common bran that’s seen in bran muffins and coffee can reduce the absorption of your thyroid medication.
In terms of Pharmacokinetics it is 40% bioavailable.
It is absorbed from the jejunum and ileum so it’s going to be important for you to adjust the doses
or be wary of the doses in patients who have had either their jejunum or ileum removed
or if they have a jejunostomy or ileostomy.
There is increased absorption during fasting and with hyperacidity states,
so your patients who are fasting during the month of Ramadan for example,
should have their dosages adjusted because they are going to actually increase the amount they absorb.
Similarly, people who have very acidic gastrointestinal milieu will have an increased absorption
so you may wanna to start those patients on a slightly lower dose.
There is decreased absorption in patients who have malabsorption syndromes,
so those patients who are suffering from this condition should be started on a higher not a lower dose.
One of the types of thyroid replacement strategies is desiccated thyroid sometimes sold as whole thyroid.
This is actually made from the dried thyroid glands of animals either beef or pork.
There is no evidence that it is superior or better, although it may be preferred by some patients.
Some doctors also prefer based on science that I don’t think it’s particularly, let’s say, well supported.
The T3 T4 balance is different in those substrates than it is in humans
and that’s why I say that I don’t think the science is very strong for its use.
Now let’s move on to the T3 analogs. We commonly refer to this as liothyronine.
Remember that T3 is between 3 to 10 times more potent than the T4 enzymes.
Most patients are converting T4 to T3 but they are going to be some patients
who don’t do that and that’s why we use the T3.
It is a faster acting shorter half-life more expensive version of thyroid.