Chapter 45: Thyroid & Parathyroid Disorders – Hormone Therapy

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Today, we're taking a really close look at endocrine pharmacotherapeutics.

Specifically,

the thyroid and parathyroid glands the body's big metabolic regulators.

This is definitely core stuff for advanced practice.

Absolutely.

Think of the thyroid as, well, the body's central metabolic engine.

It makes T3 and T4, and these hormones are just critical for basal metabolic rate, how warm we stay, cardiac output, and especially for brain development.

Really vital.

And that whole system, the HPT axis hypothalamic pituitary thyroid, that's your classic feedback loop, right?

TRH triggers TSH, TSH triggers the thyroid.

Exactly.

And then the T4 and T3 tell the pituitary to ease off negative feedback.

But clinically, the most important thing to remember is TSH.

It's just incredibly sensitive.

Right.

It changes first.

Way before anything else.

And remember the difference.

T4 is mostly what the thyroid makes, like 80%.

It's the prohormone.

It hangs around for a while, too.

The half -life is 7 to 10 days.

Okay.

Long half -life for T4.

Yeah.

And T3, that's the active form, much more potent, but only about 20 % of production, and it's gone in about a day.

Tissues convert the T4 into T3 when they need it.

That makes sense.

So T4 is the reserve, T3 is the immediate power.

If TSH is so sensitive, when would we as practitioners actually need to look at the free T4 level?

Good question.

Mostly if you suspect the problem isn't the thyroid itself, but maybe the pituitary that's secondary hypothyroidism.

Or sometimes, you know, to clarify things in really subtle subclinical cases.

But honestly, for most primary care situations, TSH tells the story.

It's the go -to for monitoring therapy.

And what level are we usually shooting for?

For younger, healthy adults, you typically want it in the lower end of the normal range, maybe 0 .5 to 2 .5 MIUL.

It does tend to creep up naturally as people get older, so context matters.

Okay.

So today's mission then.

Yeah.

Break down the drugs for when there's too little hormone hypothyroidism and too much hyperthyroidism.

And then we'll touch on the parathyroid stuff.

Too much like a plan.

All right.

Let's start with hypothyroidism.

The state of not enough hormone.

Usually this is Hashimoto's thyroiditis, right?

The autoimmune attack.

That's the most common cause, yes.

An autoimmune process damaging the gland.

But you also see it a lot after thyroid surgery or radiation therapy to the neck.

And certain drugs are notorious for causing it.

And the symptoms?

It's like everything slows down.

Exactly.

Patients feel wiped out, cold all the time,

maybe gaining weight even if they're not eating more.

Constipation is common.

Brain fog, a slow heart rate.

Long -term, if it's not treated, you worry about high cholesterol and definitely increased cardiovascular risk.

Diagnosis seems pretty lab -driven here.

Totally.

It hinges on that TSH.

If TSH is high, say over 10 MIUL, and the free T4 level is clearly low, that's overt hypothyroidism.

Pretty straightforward.

But there's also subclinical.

Right.

And that's trickier.

TSH is elevated maybe between 4 .5 and 10, but the free T4 is still hanging in the normal range.

It's like the body's working over time, the TSH is yelling, but the T4 output hasn't failed yet.

Whether to treat often depends on symptoms or if they have those thyroid antibodies.

Okay.

So the treatment for this lack of hormone is

replacement.

Levothyroxine, LT4.

That's the one.

Synthetic T4.

It's the gold standard, really.

Why T4 specifically and not T3 or a mix?

Great question.

We use LT4 because it acts just like the body's own T4.

It binds to the same receptors.

And that long half -life, the 7 to 10 days we mentioned, that makes dosing really stable, usually just once a day.

Plus, it lets the body's tissues do their own fine -tuning, converting T4 to the active T3 only as needed locally.

Using straight T3, like liophyrinine, is riskier.

You get these big peaks and troughs and levels, more potential for toxicity, LT4 is smoother, more physiologic.

Okay, dosing.

The average is about 1 .6 msg per day using ideal body weight.

But what about tricky situations like, say, the patient who just isn't getting better on what seems like the right dose?

Ah, yes.

Before you even think about increasing the dose, you have to ask about how they're taking it.

Absorption is everything with LT4.

This is the big clinical pearl, isn't it?

It absolutely is.

The number one reason for apparent treatment failure, besides non -adherence, is taking it wrong.

LT4 absorption is hugely affected by stomach acid and by binding to other things.

If they take it with breakfast or with their calcium supplement or iron or even an antacid or PPI, they might only absorb like 60 % of the dose.

So what's the rule?

The absolute rule.

Take LT4 on a completely empty stomach.

That means either a full hour before the first meal of the day or wait at least four hours after their last meal, like at bedtime.

And they need to be consistent with when they take it.

Wow, okay.

So counseling on that is critical.

You mentioned interactions, things that decrease absorption like calcium, iron, antacids, PPIs.

Right.

And you also have to watch out for drugs that speed up how the body gets rid of LT4.

Things like refampin, phenytoin, carbamazepine, these are enzyme inducers.

If someone starts or stops one of those, you'll likely need to adjust their LT4 dose big time.

Needs careful separation to usually separate LT4 by at least two hours before or six hours after these interacting meds.

And because of that long half -life, monitoring needs patience.

Exactly.

Don't check TSH too soon after starting or changing a dose.

Wait six to eight weeks.

Let it reach steady state.

And dose changes should be small, like 10 % or 20 % adjustments once they're stable.

Checking TSH every six to 12 months is usually fine.

Is there any issue with switching between, say, brand name synthroid and a generic levothyroxine?

Yeah, that's a potential issue.

There can be slight differences in bioavailability between formulations.

So ideally, patients should try to stick with the same product, whether it's a specific brand or a specific generic manufacturer if possible.

Switching could lead to TSH fluctuations.

And what happens if we overshoot the dose?

Too much LT4.

That's basically inducing hyperthyroidism.

You worry about symptoms like palpitations, anxiety, weight loss.

But the big risks are atrial fibrillation, especially in older adults.

And in post -menopausal women, overtreatment can accelerate bone loss and increase the risk of fractals.

This is why we're cautious, especially when starting therapy in older patients or those with heart disease.

How cautious?

Really cautious.

In someone with known ischemic heart disease, you might start as low as 12 .5 or 25 micrograms per day.

Start low, go slow.

Got it.

And just quickly, the extremes, mixed edema coma.

That's a medical emergency.

Life -threatening hypothyroidism.

You hit them hard and fast.

A big IV bolus of LT4, maybe 300, 500 mg, plus IV hydrocortisone, because adrenal function might also be compromised.

Also critical, hypothyroidism during pregnancy is bad news risk of miscarriage, impaired fetal brain development.

The thyroid hormone demand goes way up.

So you typically need to increase the LT4 dose by 20 to 50%, sometimes even more right at the start pregnancy.

And monitor TSH frequently, like every four weeks during the first half.

Okay.

That covers the too little side.

Let's flip to the too much hyperthyroidism or thyrotoxicosis.

The engine's running too fast.

Right.

Too much thyroid hormone floating around.

The most common reason in the U .S.

is Graves disease.

Another autoimmune condition.

Yep.

But this time, the body makes antibodies, thyroid receptor antibodies, or TRAB, that actually mimic TSH.

They bind to the TSH receptor on the thyroid and just constantly tell it to make more hormones.

So the symptoms are the opposite of hypo.

Pretty much.

Nervousness, anxiety, feeling hot all the time, sweating, weight loss, even though they might be eating more, palpitations, a fast heart rate, maybe tremors, untreated.

The big risks are atrial fibrillation again, heart failure, and also osteoporosis because of that increased metabolic rate affecting bone turnover.

Diagnosis.

Low TSH, high T4.

Exactly.

TSH will be low, often undetectable because the pituitary is trying desperately to shut down production, and free T4 will be high, but then you need to figure out why.

Is the thyroid itself overactive or is it just leaking hormone?

And that's where the RAIU test comes in.

Radioactive iodine uptake.

Precisely.

You give a tiny safe dose of radioactive iodine and see how much the thyroid gland takes up.

If uptake is high, it means the thyroid factory is actively churning out hormone that points to Graves disease or maybe a toxic nodular goiter.

If uptake is low, it means the thyroid isn't actively making hormone, it's probably inflamed or damaged and just leaking stored hormone.

That suggests thyroiditis.

And absolutely contraindicated in pregnancy, right?

Absolutely.

No radioactive iodine if there's any chance of pregnancy.

Okay, so treatment options here seem more varied.

We've got drugs,

radioactive iodine ablation, or surgery.

That's right.

Three main paths.

You can use anti -thyroid drugs or ATDs.

The goal there is usually to control the hormone production and hopefully induce remission.

About a 40 -60 % chance remission after a year or two of treatment.

Remission not necessarily cure.

Correct.

It works best for people with milder disease or smaller goiters, or those who want to avoid destroying the gland.

The other two options, radioactive iodine treatment, RAI, or thyroidectomy, surgery, are ablative.

They aim to destroy or remove the thyroid tissue.

So you intentionally make them hypothyroid?

Essentially, yes.

Because hypothyroidism, while needing lifelong treatment with LT4, is generally much easier and more predictable to manage than hyperthyroidism.

RAI is usually a single dose of radioactive iodine, often 10 -15 millicuries.

Surgery is often preferred if the goiter is really large, if there's suspected cancer, or sometimes if the patient prefers it.

Let's focus on those ATDs.

Methamazole, MMI,

and propylthiuracil, PTU.

How do they work?

They both work by blocking a key enzyme called thyroid peroxidase, or TPO.

Think of TPO as the main worker on the thyroid hormone assembly line.

It adds iodine to tyrosine residues.

MMI and PTU inhibit TPO, shutting down hormone synthesis.

Okay, they stop production.

Right.

Now, PTU has one extra trick.

It also partially blocks the conversion of T4 to the more potent T3 out in the peripheral tissues.

MMI doesn't really do that.

So why isn't PTU preferred then, if it has that extra action?

MMI seems to be the first choice, usually.

Yeah, MMI is generally preferred.

It can be dosed once a day, and its side effect profile is generally considered a bit better, with one major exception related to pregnancy.

The main reason MMI is usually first line is because PTU carries a higher risk of serious liver toxicity.

Ah, okay.

But you mentioned pregnancy.

Yes.

This is critical.

MMI has been linked to birth defects, particularly if used in the first trimester.

So PTU is actually the preferred ATD during the first trimester of pregnancy.

Often, patients are then switched back to MMI for the second and third trimesters because of that liver risk with PTU.

Let's talk about those risks.

These sound like the big red flags we need to warn patients about.

Absolutely essential counseling points.

The first one, which can happen with either drug, is granulocytosis, a severe drop in white blood cells, specifically neutrophils.

It leaves the patient incredibly vulnerable to infection.

It's rare, maybe occurs in .3 % of patients, usually within the first three months of starting the drug.

You have to tell patients, if you develop a fever, feel generally unwell, or get a sore throat while taking these meds, stop the drug and seek medical attention immediately for a blood count.

It can be life -threatening.

Okay, granulocytosis.

What's the other major risk?

Hepatotoxicity liver damage.

And this is where PTU is particularly concerning.

It carries a black box warning specifically for severe liver injury, sometimes requiring liver transplant or even causing death.

While MMI can also cause liver issues, the risk is generally considered lower and less severe than with PTU.

That's the main driver for preferring MMI outside of that first trimester of pregnancy window.

Got it.

Serious potential side effects.

While waiting for these definitive treatments to work or for remission with ATDs, what about symptom control?

Patients feel awful.

Right, you need something for the immediate symptoms.

Beta blockers are the mainstay here.

They're fantastic for slowing the heart rate, reducing palpitations, helping with tremors and anxiety.

Any specific beta blocker preferred.

Propranolol is often favored.

It's non -selective, so it hits both beta 1 and beta 2 receptors.

And like PTU, it has a slight additional benefit of weakly inhibiting that peripheral T4 to T3 conversion.

Atenolol or metaprolol can also be used if a beta 1 selective agent is needed.

Okay.

And sometimes iodine solutions are used.

Yes.

Saturated solutions of potassium iodide like leukol solution or SSKI, they can rapidly decrease hormone release and also make the thyroid gland less vascular before surgery.

But big caution, you would not use these if the patient is going to get radioactive iodine therapy because the stable iodine will flood the thyroid and prevent the radioactive iodine from being taken up.

So timing is key.

Makes sense.

Okay.

Let's shift gears quickly to the neighbors, the parathyroid glands.

Their whole job is calcium, right?

Exactly.

Four tiny glands, usually behind the thyroid, that produce parathyroid hormone, PTH.

PTH is all about raising blood calcium levels.

How does it do that?

Three main ways.

Tells the kidneys to reabsorb more calcium, tells the bones to release calcium, and crucially stimulates the kidneys to activate vitamin D.

Active vitamin D then boosts calcium absorption from the gut.

So hyperparathyroidism is too much PTH.

Correct.

Usually caused by a benign tumor, an adenoma on one of the glands, just churning out PTH inappropriately.

This leads to high blood calcium hypercalcemia.

And the diagnosis is high calcium plus high PTH.

High calcium with a PTH level that's either high or inappropriately normal.

If calcium is high, PTH should be suppressed.

If it's not, that points to primary hyperparathyroidism.

Treatment.

The only real cure is surgery to remove the overactive gland or glands of parathyrodectomy.

But what if someone can have surgery?

Then we have a pharmacologic option.

Cinecalcet.

It's a calciumimetic.

Calciumimetic.

That makes calcium.

Kind of.

It actually makes the calcium sensing receptors on the parathyroid gland more sensitive to the calcium that's already there.

So it effectively tricks the gland into thinking calcium levels are higher than they are, which strengthens the normal negative feedback, causing it to release less PTH.

Less PTH means lower blood calcium.

Interesting mechanism.

Any key things to know about using Cinecalcet?

Yes.

It needs to be taken with food to maximize absorption.

Nausea and vomiting are pretty common side effects.

And the big safety concern is that it can lower calcium too much, causing hypocalcemia.

So you need to monitor calcium levels carefully.

Okay.

And the flip side, hyperparathyroidism.

Usually happens after neck surgery, like thyroidechymia, where the parathyroids get accidentally removed or damaged.

So not enough PTH, leading to low blood calcium hypocalcemia.

Treatment for that.

Standard treatment is straightforward.

Calcium supplements, usually calcium carbonate, and an active form of vitamin D like calcitriol.

Calcitriol bypasses the need for PTH to activate vitamin D in the kidney.

For some patients who don't get good control with just calcium and calcitriol, there's also a recombinant human PTH available now, replacing the hormone directly.

Okay.

Let's try to pull this all together.

Key therapeutic differences.

For hypothyroidism, the strategy is replacement.

LT4 acts as a prohormone, aiming for stable physiologic levels.

It's all about mimicking the natural state.

Right.

Replacement.

For hyperthyroidism, the strategy with drugs like MMI and PTU is blockade.

You're actively inhibiting hormone synthesis.

Blockade.

Got it.

And for both conditions, monitoring that TSH level is absolutely central to guiding therapy.

And patient education is huge, especially the LT4 empty stomach rule and the critical warning signs for ATDs like fever or sore throat.

That LT4 rule seems like something easily missed, but crucial for efficacy.

Oh, definitely.

Consistency is key.

So we talked about LT4 dosing needs maybe decreasing as people age, but thinking about that long half -life, 7 to 10 days, and the fact that older patients often have more health issues, more medications that could interact.

Does the slow clearance and potential for accumulation actually mean we should maybe monitor TSH more often in older adults, not less, to avoid accidentally over -treating them?

That's a really interesting point.

You're right.

The balance is delicate.

Aging brings changes in metabolism, absorption, other illnesses.

While guidelines might suggest less frequent monitoring once stable, the accumulation of factors in older patients plus that long half -life, it probably argues for continued vigilance, maybe individualized frequency based on their overall clinical picture.

It's certainly not a set it and forget it situation, especially in geriatrics.

Avoiding both hypo and hyperthyroidism is critical.

A continuous balancing act indeed.

That's a great thought to leave us with.

Thank you so much for walking us through all of that.

And thank you, our listeners, for joining this deep dive into thyroid and parathyroid pharmacology.

Hopefully these insights prove valuable in your practice.