Understanding How Will I Feel After a Thyrogen Injection

October 8, 2025 •

4 min read

In clinical trials, 11% of patients reported nausea as a side effect of Thyrogen. Understanding what to expect can help you prepare, so here's a detailed look at the question: How will I feel after a Thyrogen injection?

Quick Summary

After a Thyrogen injection, many people feel fine, but some may experience temporary side effects like nausea, headache, fatigue, or flu-like symptoms. This allows patients to avoid severe hypothyroidism.

Key Points

Primary Feelings: The most common side effects after a Thyrogen injection are nausea and headache, but many patients feel fine.
Purpose: Thyrogen is used to stimulate thyroid cells for cancer testing or treatment without stopping hormone medication.
Key Advantage: It allows patients to avoid the severe symptoms of hypothyroidism associated with thyroid hormone withdrawal (THW).
Common Side Effects: Besides nausea and headache, some may experience fatigue, dizziness, vomiting, or temporary flu-like symptoms.
Administration: The standard protocol is two injections into the buttock muscle, 24 hours apart.
Serious Risks (Rare): Be aware of rare but serious risks like allergic reactions, stroke, and sudden tumor enlargement, and seek immediate medical help if symptoms occur.
Preparation: Preparation usually involves a low-iodine diet and staying well-hydrated before the injections.

What is Thyrogen and Why Is It Used?

Thyrogen, the brand name for thyrotropin alfa, is a man-made form of thyroid-stimulating hormone (TSH). It is used in the management of well-differentiated thyroid cancer after a patient has had their thyroid gland removed (thyroidectomy). Its primary purpose is to stimulate any remaining thyroid tissue, including potential cancer cells, to absorb iodine. This "wakes up" the cells, making them visible on scans and receptive to treatment with radioactive iodine (RAI).

Historically, the only way to achieve high TSH levels was through thyroid hormone withdrawal (THW). This involved stopping thyroid hormone replacement medication for several weeks, which induces a state of profound hypothyroidism. Patients often experience significant side effects during THW, including fatigue, weight gain, depression, and cognitive slowing. Thyrogen provides a major advantage by allowing patients to continue taking their thyroid hormone medication, thus avoiding the debilitating symptoms of hypothyroidism while still preparing their body for testing or treatment.

How Will I Feel? Common and Less Common Side Effects

For many patients, the experience of receiving Thyrogen injections is uneventful, with some reporting only minor soreness at the injection site, similar to any other shot. However, a range of side effects can occur. It's helpful to categorize these by how frequently they are reported.

Most Common Side Effects

The most frequently reported side effects in clinical studies are generally mild and transient:

Nausea: This is the most common side effect, affecting about 11% of patients.
Headache: The second most common reaction.
Fatigue, Weakness, or Lack of Energy (Asthenia): A feeling of tiredness or weakness is also common.
Dizziness: Some people may feel light-headed or dizzy.
Vomiting: Less common than nausea, but still a notable side effect.

Less Common and Flu-Like Symptoms

Some individuals report experiencing temporary flu-like symptoms, which typically resolve within 48 hours. These can include:

Fever (over 100°F/38°C)
Chills or shivering
Muscle or joint pain (myalgia/arthralgia)
Runny or stuffy nose
Sore throat

Rare But Serious Side Effects

Though infrequent, it is crucial to be aware of more serious potential side effects. You should seek immediate medical attention if you experience any of the following:

Allergic Reactions: Symptoms like hives, skin rash, itching, swelling of the face, lips, or tongue, and difficulty breathing.
Sudden Tumor Enlargement: In rare cases, remaining thyroid tissue or metastases can swell rapidly and painfully, potentially causing symptoms like breathing difficulty, vision changes, or pain depending on their location.
Stroke: There have been rare post-marketing reports of stroke, particularly in young women with other risk factors like smoking or use of oral contraceptives. Symptoms include sudden numbness or weakness (especially on one side), severe headache, confusion, or trouble with vision or balance.
Thyrogen-Induced Hyperthyroidism: In patients with a significant amount of remaining thyroid tissue, Thyrogen can cause a temporary but significant spike in thyroid hormone levels.

Thyrogen vs. Thyroid Hormone Withdrawal (THW)

The primary benefit of using Thyrogen is the significantly improved quality of life compared to undergoing thyroid hormone withdrawal. Clinical studies using the SF-36 quality of life survey showed that patients on THW experienced significant negative impacts across all domains, while those receiving Thyrogen had little change from their baseline.


Feature	Thyrogen (thyrotropin alfa)	Thyroid Hormone Withdrawal (THW)
Patient Feeling	Generally feel well; may have transient nausea, headache, or fatigue.	Experiences symptoms of severe hypothyroidism: extreme fatigue, depression, weight gain, brain fog, cold intolerance.
Procedure	Two injections into the buttock muscle, 24 hours apart.	Stop taking thyroid hormone medication (e.g., levothyroxine) for 4-6 weeks.
Quality of Life	Maintained; patients can continue most normal daily activities.	Significantly impaired; often requires taking time off work and limits daily function.
TSH Elevation	TSH levels are elevated for a short, controlled period.	TSH levels rise slowly over several weeks and remain high.
Common Side Effects	Nausea (11%), headache (6%).	Signs and symptoms of hypothyroidism are the intended effect and are widespread.

The Thyrogen Protocol: What to Expect

The typical process involving Thyrogen injections follows a set schedule over several days:

Preparation: Your doctor will likely ask you to follow a low-iodine diet for about one to two weeks before your first injection to help make any thyroid cells more absorbent of the radioactive iodine. It's also important to be well-hydrated.
Day 1 (e.g., Monday): You receive your first 0.9 mg injection of Thyrogen into the buttock muscle.
Day 2 (e.g., Tuesday): You receive your second and final injection, 24 hours after the first.
Day 3 (e.g., Wednesday): You will take your dose of radioactive iodine (RAI), typically 24 hours after the last Thyrogen injection.
Day 5 (e.g., Friday): A blood draw is performed to test your thyroglobulin (Tg) level, which is a cancer marker. This is done about 72 hours after your final injection. A whole-body scan (WBS) may also be done 48 hours after the RAI dose to look for any remaining thyroid cells.

Conclusion

For most people, the answer to "How will I feel after a Thyrogen injection?" is "mostly like yourself." The most common side effects are mild and short-lived, such as nausea and headache. The development of Thyrogen marked a significant advancement in thyroid cancer care, allowing patients to prepare for crucial diagnostic tests and treatments without enduring the harsh and prolonged side effects of thyroid hormone withdrawal. While rare but serious risks exist, the overall patient experience is far superior. Always discuss any concerns with your healthcare provider, who can give you personalized advice based on your medical history.

For more information from the manufacturer, visit: Thyrogen® Official Patient Site

Frequently Asked Questions

The most common side effects reported in clinical studies are nausea and headache. Other common effects include fatigue, dizziness, and vomiting.

Thyrogen is given as two 0.9 mg injections into the buttock muscle, with the injections spaced 24 hours apart.

No, a primary benefit of Thyrogen is that you can continue taking your thyroid hormone replacement medication, helping you avoid the symptoms of hypothyroidism.

Some patients have reported transient (lasting less than 48 hours) flu-like symptoms, which can include fever, chills, muscle aches, and fatigue.

Thyrogen is generally associated with a much better quality of life than THW because it avoids the debilitating symptoms of induced hypothyroidism, like extreme fatigue and depression.

Yes, although rare, serious side effects can occur. These include stroke, sudden and painful tumor enlargement, and severe allergic reactions. You should seek immediate medical help if you experience symptoms like sudden weakness, severe headache, or trouble breathing.

Yes, most patients are required to follow a low-iodine diet for one to two weeks before the first injection to increase the effectiveness of the subsequent radioactive iodine scan or treatment.