Diabetes and Thyroid Medicines

Introduction

Diabetes medicines and thyroid medicines can look similar at the pharmacy counter. Both are usually taken every day, refilled for a long time, and named on labels you will see often. They do very different jobs, though.

Glucose-lowering medicines help keep blood sugar in a workable range. Thyroid replacement supplies a hormone that the thyroid gland is not making enough of.

As a technician, you need to tell these medicines apart and read insulin strength correctly. You also need to notice when an instruction belongs to one product only. This lesson teaches all three, and it shows when a situation belongs to the pharmacist rather than to you.

What you will be able to do

• Explain what metformin, insulin and levothyroxine each do, and tell glucose-lowering medicines from thyroid replacement.
• Sort common insulin brands into their action groups and explain why the groups are not interchangeable.
• Read insulin concentration as units per milliliter and convert between units and volume, with a check.
• Spot instructions that belong to one product, such as storage days, pump use, release form or brand, and avoid applying them to a whole class.
• Recognize when to route a situation to the pharmacist instead of acting on it yourself.

Before you start: You need everyday reading and arithmetic, including dividing by 100. If reading a label's name, strength and form is new to you, see Read a medicine identity before learning a name. For unit-cancelling arithmetic, see Metric conversions and dimensional analysis.

Two different problems, two different jobs

Start with what is going wrong in the body. Then the medicine's job makes sense.

Glucose is sugar in the blood. The body uses it for energy, but too much in the blood over time can lead to heart disease, kidney problems, nerve damage and eye problems.

Insulin is a natural hormone that controls how much glucose stays in the blood. It helps body cells take glucose in.

Diabetes is a long-term condition in which blood glucose is not controlled normally. There are two main types:

• In type 1 diabetes, the body does not produce insulin. People with type 1 depend on insulin injections every day.
• In type 2 diabetes, the body does not use insulin normally, or does not make enough of it.

Thyroid hormone is made by the thyroid gland in the neck. Without enough of it, the body cannot function properly. Hypothyroidism means the thyroid gland is not producing enough thyroid hormone.

For each medicine in this lesson, ask two questions. What is missing or not working? What does this medicine change?

Metformin: helping the body handle glucose

Plain-English takeaway: Metformin lowers blood sugar, but it is not insulin. It helps the body manage glucose better.

Metformin works in three ways:

• It lowers the amount of glucose the liver makes.
• It lowers the amount of glucose absorbed from food.
• It increases the body's response to the insulin the body already has.

Metformin belongs to a drug family called biguanides. Here, "biguanide" is simply the family name that groups metformin.

Metformin treats type 2 diabetes. It can be used alone or with other medicines, including insulin. It is not used to treat type 1 diabetes. Metformin does not supply insulin, and a person with type 1 makes none, so that person depends on insulin itself. Metformin controls diabetes but does not cure it.

Worked example. A prescription reads "metformin 500 mg tablet." You can recognize three things:

• It is a glucose-lowering medicine.
• It is a biguanide.
• Its labeled use is type 2 diabetes.

You may hear it linked to the brand Glucophage. MedlinePlus marks Glucophage as no longer on the market, with generic alternatives possibly available. Learn that brand as a historical association, not as a product you should expect on the shelf.

Metformin can also hide inside a combination brand. For example, Janumet contains metformin plus sitagliptin. One brand name can mean two active ingredients.

Changed case: same ingredient, different formulation. Now the prescription reads "metformin extended-release." The ingredient is the same, but the product behaves differently.

• Regular tablets: usually taken with meals two or three times a day.
• Extended-release tablets: usually taken once a day with the evening meal. They must be swallowed whole, never split, chewed or crushed. A patient may notice the empty tablet shell in their stool. This does not mean they missed the dose.

The extended-release tablet is built to release the medicine slowly, so its directions differ from the regular tablet's.

Tempting mistake: "It's all metformin, so the directions are the same." The ingredient does not tell you how the product releases the drug. The formulation does.

Insulin: supplying the hormone itself

Plain-English takeaway: Insulin products supply insulin directly. They are grouped mainly by how fast they start working and how long they last.

Premixed insulins combine two groups in one product. In the mixtures described in FDA's emergency guidance, which combine intermediate-acting insulin with short- or rapid-acting insulin, the first number is the intermediate-acting share and the second is the short- or rapid-acting share. So for those mixtures, "70/30" means each dose is 70% intermediate-acting and 30% short- or rapid-acting. Do not assume every premixed product follows this pattern. Mixtures differ, so check the exact ingredients on the product's label.

Worked example: why timing separates the groups. Rapid-acting insulin starts working sooner than regular insulin. That is why FDA guidance places rapid-acting insulin closer to the meal.

Suppose rapid-acting insulin is injected as early as regular insulin would be. The insulin may start lowering blood sugar before food arrives. That risks dangerously low blood sugar, called hypoglycemia. The timing difference comes directly from the speed difference.

Products within the same group can still differ in their exact labeled timing. Recognizing the group tells you which way the timing leans. It does not give you a schedule to hand a patient. The prescriber sets the dosing, and the prescribed product's label gives the timing.

Changed case: one letter changes the group. Humulin R and Humulin N share a brand family, but they are in different action groups:

• R is regular, a mealtime insulin.
• N is intermediate-acting, a background insulin.

Humalog and Humulin look alike too, but Humalog is rapid-acting. A shared brand family name does not mean a shared product. For more on names that differ by a letter or two, see High-alert medicines and similar names.

Shared use does not authorize substitution. Lantus and Humulin N both provide background insulin, so it is tempting to call them interchangeable. FDA's emergency guidance, written for disasters when a patient's usual insulin is unavailable, shows why they are not:

• Moving between a once-daily long-acting insulin and an intermediate-acting insulin can change the dosing schedule, including how many injections are given each day.
• FDA says to always switch insulin under a doctor's care, with close blood sugar monitoring, and notes that dose adjustment may be needed during the transition.

So even in an emergency, the schedule may change and medical supervision is expected. Choosing the product and working out any change belong to the prescriber, not the technician. In everyday pharmacy work, substituting a different medicine for the same purpose is called therapeutic substitution. The PTCE content outline lists it as an issue that requires the pharmacist. A shared purpose is never a technician's permission to swap.

Levothyroxine: replacing a missing hormone

Plain-English takeaway: Levothyroxine is a man-made version of thyroid hormone. It replaces hormone the thyroid gland is not producing.

Key facts:

• Class: hormones.
• Uses: hypothyroidism. It is also used along with surgery and radioactive iodine to treat thyroid cancer.
• Brand names include Synthroid, Levoxyl, Unithroid, Tirosint and Levo-T. Levothroid is marked as no longer on the market.
• Forms: tablet and capsule, taken by mouth.

Worked example: the cause-and-effect chain.

1. The thyroid makes too little hormone.
2. The body slows in many ways. Symptoms can include tiredness, low energy, increased sensitivity to cold, constipation, weight gain and dry skin.
3. Levothyroxine supplies the missing hormone.
4. When taken correctly, the symptoms reverse. This can take several weeks.

Levothyroxine controls hypothyroidism but does not cure it, so patients continue it even when they feel well. It is usually taken once a day on an empty stomach, 30 minutes to 1 hour before breakfast.

Levothyroxine carries a warning that it must not be used to treat obesity or cause weight loss. It replaces a missing hormone. It is not a general metabolism booster.

Changed case: same morning, add calcium. Now suppose the patient also takes calcium carbonate, an antacid and calcium supplement, at the same time as the levothyroxine.

The levothyroxine labeling explains the problem. Calcium carbonate may bind levothyroxine into a chelate, a combined form that does not dissolve. Ferrous sulfate, an iron supplement, likely forms a similar complex. Either way, less levothyroxine is absorbed.

Less absorbed hormone means less replacement, so hypothyroid symptoms may return. The source instruction is to keep these products at least 4 hours apart from levothyroxine.

Qualifications:

• The labeling names these specific products, calcium carbonate and ferrous sulfate. Do not turn this into a rule you apply on your own to every product.
• Interactions belong to the pharmacist. Your job is to notice the combination and route it.

Changed case: same ingredient, different brand. MedlinePlus advises patients not to switch brands without talking to their doctor or pharmacist, because each brand contains a slightly different amount of medication. Two products with the same ingredient and the same printed strength are not automatically interchangeable for this patient.

Changed case: tablet versus capsule.

• Tablet: if a patient cannot swallow it, the tablet can be crushed and mixed in 1 to 2 teaspoons of water only. It is not mixed with food or soybean infant formula, and the mixture is given right away.
• Capsule: swallowed whole, not chewed or crushed.

The crushing instruction belongs to the tablet. It does not carry over to the capsule.

Question 1

A customer has metformin extended-release and levothyroxine on their profile. They ask, "Aren't these both for metabolism? Couldn't the thyroid pill help my blood sugar too?" Explain which problem each medicine targets and why one cannot stand in for the other.

Question 2

A coworker shelves NovoLog next to Novolin R and says, "Both are mealtime insulins, so they're basically the same thing." What fact distinguishes them, and why does it matter?

Insulin units are not milliliters

Plain-English takeaway: A unit measures how much glucose-lowering effect is present. A milliliter (mL) measures how much liquid is present. Concentration connects the two: it tells you how many units are in each mL.

The insulin unit was created because early insulin batches varied in strength. A unit was defined as the amount of insulin needed to produce a standard drop in blood sugar. It is a measure of biological effect, not of volume.

FDA's example shows this clearly. One unit equals the activity in 0.0347 mg of human insulin but 0.0363 mg of insulin glargine. Different insulins need slightly different weights to produce the same effect. A unit is anchored to effect, not to a fixed amount of liquid or powder.

FDA's definition of drug strength reflects this. Strength can be expressed in units of activity per volume, such as units/mL, rather than only in milligrams.

Common insulin is U-100, meaning 100 units/mL. Concentrated insulin packs more units into the same liquid. For example, a U-200 product contains 200 units/mL.

Worked example: from units to volume

Fictional practice case. Every number is supplied here; this is not a treatment instruction.

An order calls for 12 units. The product is 100 units/mL. How much liquid is that?

1. Set up the calculation: volume = dose in units ÷ concentration in units per mL.
2. Calculate: 12 units ÷ 100 units/mL = 0.12 mL.

The "units" cancel, leaving mL. That tells you the setup is right.

Check 1, reverse the calculation: 0.12 mL × 100 units/mL = 12 units. ✓

Check 2, judge the size: 12 units is far less than the 100 units in one mL, so the answer must be far less than 1 mL. ✓

Changed case: double the concentration

Keep the dose at 12 units, but use a 200 units/mL product:

12 units ÷ 200 units/mL = 0.06 mL

The dose is still 12 units. Each mL now holds twice as many units, so half the liquid delivers the same effect. Doubling the concentration halves the volume; it does not change the dose.

Tempting mistakes

• "U-200 means a bigger dose." No. The dose is set in units by the prescriber. The concentration only changes how much liquid carries those units.
• "12 units means 12 mL." At 100 units/mL, 12 mL would hold 1,200 units, one hundred times the intended dose. The units-to-mL setup and the size check both catch this error.
• Moving insulin out of a pen. An insulin pen's dose window shows the number of units selected for the insulin inside that pen. Patient-safety guidance says never to use a syringe to take insulin out of an insulin pen, especially concentrated insulin, because this could result in a wrong dose. FDA's emergency guidance also says Awiqli must not be drawn from the pen into a syringe.

The arithmetic shows why concentrated insulin raises the stakes. A tiny extra 0.01 mL of liquid holds 1 unit at 100 units/mL but 2 units at 200 units/mL. The same small measuring slip carries twice the effect.

Total units in a container

Fictional practice case. Suppose a pen holds 3 mL at 100 units/mL:

3 mL × 100 units/mL = 300 units

Change only the concentration to 200 units/mL:

3 mL × 200 units/mL = 600 units

The same-size pen holds twice the units. For more practice with this kind of arithmetic, see Concentrations, ratios and dilutions.

Question 3

Fictional practice case. An order calls for 30 units. Find the volume for a 100 units/mL product and for a 200 units/mL product. Then check each answer.

Correct arithmetic is not permission

Fictional practice case. An order is for a Humalog U-100 pen. The shelf has only Humalog U-200. A technician correctly works out that the same number of units would take half the volume. Can they fill with U-200?

No. The arithmetic answers one question: how much liquid carries this many units? It does not answer whether this is the right product for this patient.

U-200 is a different product with a different concentration. It also carries its own restrictions. For example, FDA's emergency guidance says Humalog U-200 must not be used in insulin pumps, and you may not know how this patient uses their insulin.

Choosing a different product is a decision for the pharmacist and prescriber. Your correct math is useful information to hand to the pharmacist, but it does not authorize the change.

Instructions that belong to one product

Plain-English takeaway: A product-specific instruction comes from one product's own labeling. A class-wide rule is true for a whole group of medicines. Many instructions that sound general are actually product-specific.

Storage days differ by product

FDA's emergency guidance, written for disasters when usual supplies or refrigeration may not be available, draws on manufacturers' labels to describe a common pattern for insulin:

• Store in a refrigerator at about 36°F to 46°F. Unopened insulin stored this way keeps working until its expiration date.
• Vials and cartridges, opened or unopened, can stay unrefrigerated at 59°F to 86°F for up to 28 days.
• Insulin that has been diluted or removed from the manufacturer's original vial should be thrown away within two weeks.
• Do not use insulin that has been frozen.

But 28 days is not universal. One patient-education table lists different in-use periods for different pens:

• Lantus SoloStar: 28 days
• Levemir FlexTouch: 42 days
• Humulin N KwikPen: 14 days

Awiqli is different again. FDA's emergency guidance says its pens may be kept at room temperature below 86°F for up to 12 weeks in total, whether opened or unopened.

Some instructions point in opposite directions

General pen guidance from one cancer center says that once a pen has been used, it should never go back in the refrigerator. Awiqli's guidance says an in-use pen may be refrigerated, with the same 12-week limit either way.

These two instructions conflict. They are reconciled by following the specific product's own instructions.

Pump rules are product- and device-specific

These points also come from FDA's emergency guidance:

• Insulin in a pump's infusion set should be thrown away after 48 hours, unless the pump's instructions say otherwise.
• Some products allow longer: NovoLog, Kirsty and Humalog U-100 specify up to 6 to 7 days in the reservoir. Fiasp PumpCart specifies 4 days.
• Not every rapid-acting insulin is approved for pumps. Merilog is not currently labeled for pump use. Humalog U-200 and Lyumjev U-200 must not be used in pumps.
• Awiqli must not be used in a pump or mixed with other insulins.

Emergency guidance stays emergency guidance

FDA's switching and storage advice is written for disasters, when a patient's usual insulin or refrigeration is unavailable. Even then, FDA says to switch under a doctor's care with close monitoring. It is not a general substitution or storage rule for routine pharmacy work.

Non-insulin examples

The same principle applies beyond insulin:

• Metformin extended-release must be swallowed whole; the regular tablet has different directions.
• A levothyroxine tablet can be crushed in water, but the capsule cannot.
• Levothyroxine brands differ slightly in the amount of medication they contain.

The decision habit

When an instruction includes a number of days, a device, a form or a brand, ask: "Whose label is this from?" Then check that product's labeling.

For more on storage and temperature handling, see Stability, storage and temperature excursions.

Question 4

A patient picking up a Humulin N KwikPen says, "I know insulin is good for 28 days out of the fridge." What is wrong with this generalization, and what should you do?

Recognize, then route

Three different roles meet at the pharmacy counter:

• The prescriber chooses the medicine and the dose. These are clinical decisions.
• The pharmacist handles therapeutic substitution, drug interactions, adverse drug events and questions about how a patient is taking a medicine. The PTCE content outline lists these as issues that require pharmacist intervention.
• The technician recognizes the signal and routes it to the pharmacist promptly. Recognizing a signal is not the same as deciding what to do about it.

For a fuller treatment, see Recognize when the pharmacist must intervene.

An adverse event does not prove cause

An adverse event is a harmful or unwanted event that happens while a person is taking a medicine. It may or may not be caused by the medicine.

Suppose a patient recently started metformin and now reports nausea and stomach pain. Nausea and stomach discomfort are listed as metformin side effects. But nausea, vomiting and stomach pain also appear among the warning symptoms of lactic acidosis, a rare but life-threatening condition linked to metformin, for which MedlinePlus tells patients to stop the medicine and call their doctor immediately. The symptoms could also come from an unrelated illness.

The timing alone does not tell you which explanation is true. So the technician should neither reassure ("that's normal") nor conclude ("metformin caused it"). The right move is to route the report to the pharmacist promptly.

Serious side effects can be reported to FDA's MedWatch program. A report is a safety signal to investigate, not proof that the medicine caused the problem. See Report errors and improve the system.

Federal sources do not erase state rules

FDA labeling and FDA guidance describe products and how to use them safely. They do not replace the pharmacy rules of the state where a pharmacy operates, or that pharmacy's own procedures. State rules can add their own requirements.

FDA's own Orange Book shows the limits of a federal product description. It says products rated as therapeutically equivalent can be substituted with the expectation of the same effect. In the same passage, it notes that a prescriber may require a specific product when differences matter for a particular patient.

This lesson does not teach any one state's substitution rules.

Independent practice

Question 5

Classify each product as a glucose-lowering medicine or thyroid replacement. For insulins, name the action group.

Tresiba, Novolin R, Fiasp, Synthroid, metformin, Janumet

Question 6

Fictional practice case. A 10 mL vial is labeled 100 units/mL.

1. How many units does the vial hold?
2. How much volume contains 25 units?
3. A trainee writes "25 mL." How would a size check catch this error?

Question 7

A patient filling levothyroxine mentions they take calcium carbonate tablets with it every morning "to save time." What is the concern, and what should the technician do?

Question 8

A levothyroxine refill came from a different brand than the patient's usual one. The patient now reports nervousness, trouble sleeping and a racing heartbeat, and says, "The new brand did this, so I'll just stop taking it." What should the technician recognize, and what should they do?

Question 9

A patient uses Humalog U-100 in an insulin pump. They ask to switch to Humalog U-200 "because the pen lasts longer." The arithmetic for the same units is easy. Why can't the technician handle this request?

Question 10

A coworker says, "Lantus and Humulin N are both background insulins, and FDA has guidance on switching between them. So we can swap one for the other when we're out of stock." Identify the errors in this reasoning.

What to remember

• Metformin is a biguanide used for type 2 diabetes. It lowers the glucose the liver makes, lowers the glucose absorbed from food, and improves the response to insulin. It is not insulin and is not used for type 1.
• Insulin supplies the hormone itself. The action groups are rapid, short (regular), intermediate, long-acting and once-weekly, and their timing differs. A shared brand family or shared purpose does not make two insulins interchangeable.
• Levothyroxine replaces thyroid hormone in hypothyroidism. It is usually taken on an empty stomach and kept at least 4 hours apart from calcium carbonate and ferrous sulfate. Brands are not switched without the doctor or pharmacist.
• Units measure effect; mL measure liquid. Volume = units ÷ units per mL. Check by reversing the calculation and judging the size. Higher concentration means less liquid for the same dose, and never withdraw insulin from a pen with a syringe.
• Same ingredient does not mean same instructions. Storage days, pump approval, release form and brand all come from the specific product's labeling.
• Recognize, then route. Correct math does not authorize dispensing, an adverse event does not prove cause, and federal sources do not replace state rules. Substitution, interactions and possible side effects go to the pharmacist.