Medical Oncologist
Treatment side effects
Underactive Thyroid (Hypothyroidism) — Caused by Cancer Drugs
Targeted therapies and immunotherapy can slow down your thyroid gland. For most people this is manageable with a daily tablet, and it does not usually mean stopping your cancer treatment. But certain symptoms need urgent attention the same day.
Medically reviewed by Dr. T. Raghavender Reddy, Medical Oncologist, MBBS · DM (Medical Oncology) · MD (Radiation Oncology) · Last reviewed August 2026
- A known side effect — Thyroid problems are among the more common endocrine side effects of checkpoint inhibitors and some targeted therapies.
- Usually manageable — Most people with drug-induced hypothyroidism are treated with a daily thyroid hormone tablet and continue cancer treatment.
- Blood tests catch it early — Your team monitors thyroid function at regular intervals, often finding changes before you notice symptoms.
- Some symptoms need urgent care — Severe confusion, inability to stay awake, or a very slow pulse alongside other hypothyroid symptoms need same-day emergency assessment.
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Targeted therapy and immunotherapy can slow the thyroid gland, causing fatigue, weight gain, and feeling cold. For most people this is manageable with a daily thyroid hormone tablet, and it does not usually mean stopping your cancer treatment. NCCN and ASCO guidance recommends routine thyroid monitoring throughout treatment so that changes are caught early.
If a symptom below applies to you, do not wait for a callback. Call 1800-202-8726 or go to your nearest emergency department.
What is hypothyroidism from cancer drugs, and is it dangerous?
When targeted therapies or checkpoint inhibitors affect the thyroid gland, it stops producing enough thyroid hormone. Thyroid hormone controls your metabolism, energy, body temperature, and how your organs function.
For most people, drug-induced hypothyroidism is not dangerous when it is identified and treated. Your team checks for it through routine blood tests — often catching changes before you notice anything.
The risk comes when it goes unrecognised and becomes severe over weeks or months. Reporting new symptoms promptly is what prevents that.
What can I do at home while my team monitors my thyroid?
- Take levothyroxine at the same time each day, on an empty stomach, and separately from other tablets — consistency matters more than the exact hour.
- Tell your team about every supplement, Ayurvedic preparation, or over-the-counter tablet you are taking, because some interfere with levothyroxine absorption.
- Keep a brief daily note of your energy, mood, and weight — this is far more useful at your next appointment than trying to recall how you felt three weeks ago.
- Do not change your levothyroxine dose on your own, even if you feel better or worse than expected.
- Do not skip blood test appointments while your dose is being adjusted — thyroid levels need checking at each interval your team sets.
- Wear layers and keep warm at home if cold intolerance is affecting you — this is a real symptom, not just sensitivity.
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How do I know whether my symptoms need a call today?
| Symptom | Mild — mention at next visit | Moderate — call your team this week | Severe — call or go today |
|---|---|---|---|
| Fatigue | More tired than usual but managing daily tasks | Too tired to complete most daily activities | Cannot get out of bed or stay awake |
| Feeling cold | Feeling chilly more often than before | Cold most of the time and hard to warm up | Shivering uncontrollably, unable to warm up at all |
| Weight change | Slight, gradual increase | Noticeable gain over a few weeks with no change in diet | Rapid gain with swelling of face, hands, or legs |
| Mental clarity | Minor forgetfulness or slightly slow thinking | Difficulty concentrating on most tasks | Confusion, unable to follow a conversation or be woken easily |
| Heart rate | No change noticed | Noticeably slower than your usual | Chest pain or feeling faint alongside a slow pulse |
| Typically starts | Weeks to months into treatment — gradual onset is most common | Can worsen slowly if unrecognised | Severe presentations are rare and usually follow prolonged untreated hypothyroidism |
Questions you are likely to have
Will I need thyroid medication for the rest of my life?
Not necessarily, though it depends on which drug caused the problem. Hypothyroidism caused by immune checkpoint inhibitors often persists and may require long-term thyroid hormone replacement even after cancer treatment ends. Hypothyroidism from some targeted therapies can improve after the dose is adjusted or the drug is stopped. Your endocrinologist will give a clearer answer once your thyroid function has stabilised — it is a reasonable question to ask directly at your next review rather than waiting for them to raise it.
Will this affect my cancer treatment?
In most cases, no. Thyroid dysfunction caused by a checkpoint inhibitor can actually be a sign that the immune system is active — which is what these drugs are designed to do — and it is usually managed alongside treatment rather than by stopping it. Your oncologist will weigh the severity of your thyroid symptoms against how well your cancer treatment is working. Reporting symptoms early keeps more options available, including the option of continuing without interruption.
Can diet or supplements help my thyroid?
A balanced diet supports general wellbeing, but there is no food or supplement that replaces thyroid hormone when the gland is underproducing. Some supplements — including high-dose iodine, kelp tablets, and certain herbal preparations — can actually interfere with thyroid function or with levothyroxine absorption. Tell your team about everything you are taking before you add or change anything, rather than after. This includes preparations your family may suggest with good intentions.
My hair is falling out. Is that the hypothyroidism or the cancer drug?
Both are possible, and distinguishing them matters for deciding what to do. An underactive thyroid slows cell turnover, which affects hair growth and texture. Some targeted therapies and chemotherapy drugs also directly affect hair follicles through a separate mechanism. Treating the hypothyroidism often helps hair-related symptoms over time, but recovery is slow and may take several months. Tell your team about the hair loss specifically — it helps them assess which cause is dominant and whether your thyroid dose needs adjusting.
Is it safe to fast during Ramadan or for a religious observance while taking levothyroxine?
Fasting does not generally stop you from taking levothyroxine, because the tablet is taken with water only and does not break a fast in most interpretations. What matters is consistency — the same time each day, not with food or other tablets. Discuss the timing with your oncology nurse or doctor before any planned fasting period so your schedule is agreed in advance. Do not skip doses or change the timing on your own during the fast, as thyroid levels take weeks to restabilise after an interruption.
Did you know?
Thyroid dysfunction is among the most frequently reported endocrine side effects of immune checkpoint inhibitors, and it often develops gradually over weeks — which is why routine blood tests catch it before symptoms become noticeable.
Because tiredness, weight gain, and feeling cold overlap so much with other treatment effects, hypothyroidism is regularly missed without those scheduled tests.
Source: ESMO Clinical Practice Guidelines: Toxicities of Immunotherapy
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Common questions
Frequently asked questions
How will I know if my thyroid is being affected — will I feel it?
Not always at first. Hypothyroidism from cancer drugs tends to develop slowly, and many people notice the blood test result before they connect their fatigue or weight gain to a thyroid problem. This is why your team checks thyroid function at regular intervals during treatment rather than waiting for you to report symptoms. If you do notice increasing tiredness, weight gain, feeling cold, or difficulty concentrating, tell your team rather than assuming it is simply the treatment.
Can I take levothyroxine at the same time as my targeted therapy tablet?
Timing matters with levothyroxine, and some drugs do affect absorption. The general guidance is to take it on an empty stomach and separately from other tablets, but your specific cancer drug regimen may need a slightly different arrangement. Ask your pharmacist or oncology nurse for written instructions on timing rather than working it out yourself. Poor absorption means your thyroid level stays low even though you are taking the medication — and that can look like an undertreated thyroid when the real issue is a timing problem.
My blood test showed a high TSH. What does that mean?
A high TSH means your pituitary gland is working harder to stimulate a thyroid that is not producing enough hormone. It is the first marker to change when hypothyroidism is developing, often before you feel anything different. Your team will decide whether the level warrants starting or adjusting treatment. A mildly elevated TSH with no symptoms may be monitored for a short period first; a clearly elevated TSH with symptoms is usually treated sooner. Ask your team what number they are seeing and what they plan to do next.
Do I need to see an endocrinologist separately?
Your oncology team can manage mild to moderate hypothyroidism directly, and many cases are handled within the cancer team without a separate referral. If your thyroid levels are difficult to stabilise, if you have a pre-existing thyroid condition, or if your symptoms are significantly affecting your quality of life, an endocrinology referral is reasonable to ask about. Both teams working in parallel is the most common arrangement when the situation is more complex.
Will my thyroid recover once I finish cancer treatment?
It depends on which treatment caused the problem. Hypothyroidism from some targeted therapies may improve after the drug is stopped or the dose is changed. Hypothyroidism caused by immune checkpoint inhibitors more often persists beyond the end of treatment, and many people continue to need thyroid hormone replacement long term. Your thyroid function will be re-assessed after treatment ends, and your team will advise based on those results whether to continue, reduce, or stop thyroid medication.