Thyroid Medication Interaction Checker
Written and medically reviewed by Dr. Westin Childs, D.O. Last reviewed: August 28, 2026.
How To Use This Checker
Enter your thyroid medication in the highlighted box at the top, or leave it blank if you are not on one. Plenty of this matters before anyone starts treatment, and some of it is the reason treatment gets started.
Then list everything else. Prescriptions, supplements, infusions, injections and patches. Brand names work as well as generics, so Keytruda matches as readily as pembrolizumab. The checker recognizes more than 2,500 names.
Most people have only ever been told one thing about this: keep the calcium and the iron a few hours away from your levothyroxine. That is absorption, and it is real, but it is the smallest part of what medications do to your thyroid. It is also the only part that spacing pills apart will fix. This checker looks at six other things.
The Six Things We Check On Every Medication
#1. Its impact on the gland itself
Some drugs damage or suppress the thyroid gland directly. This is the group that changes what dose you need, or whether you need a dose at all. Checkpoint inhibitors, tyrosine kinase inhibitors, interferon, interleukin-2, alemtuzumab, amiodarone, lithium and iodine loads all belong here. Many produce a short overactive phase before settling into a permanent underactive one.
#2. Its impact on T4 to T3 conversion
T4 is storage hormone. T3 is the active one. Glucocorticoids, propranolol, amiodarone and propylthiouracil can slow the step in between. This produces the most frustrating pattern in thyroid medicine: a normal TSH, a normal free T4, a low free T3, and a patient who still feels terrible.
#3. Its impact on your TSH
Several drugs move TSH without touching your thyroid. Steroids, dopamine agonists, somatostatin analogues, metformin and opioids push it down, which makes your labs read better than you feel. Metoclopramide, antipsychotics and lithium push it up, which can trigger a dose increase you never needed.
#4. Its impact on thyroid binding globulin, the carrier protein
Thyroid hormone travels through your blood attached to a carrier protein. Oral estrogen and the combined contraceptives raise that protein, so more of your hormone is bound, less is usable, and you need more medication. Androgens and high dose steroids do the reverse. Transdermal estrogen does neither, which is worth knowing before a dose gets changed.
#5. Its impact on how fast you clear thyroid hormone
Phenytoin, carbamazepine, phenobarbital, rifampin and St John’s wort push your liver to process thyroid hormone faster. The prescription has not changed but the dose stops holding, and TSH drifts up while everyone wonders why.
#6. Its impact on the blood test itself
Biotin does nothing at all to your thyroid. It interferes with the assay most laboratories use, so TSH reads falsely low and free T4 and free T3 read falsely high. You look overactive on paper when you are not, and doses have been cut over exactly that.
What This Tool Will Not Do
It will not tell you that a medication is harming you, and it will not tell you to stop anything. Most of the drugs flagged here are treating something that matters more than the inconvenience of a thyroid adjustment. Somebody on pembrolizumab is not going to stop pembrolizumab.
What it will do is tell you what to watch for and what to ask to have tested, so that a thyroid problem caused by a medication gets recognised as one instead of being dismissed as fatigue from the underlying condition.
It is also not a timing tool. If what you actually need is a daily schedule, use the Thyroid Medication Timing Optimizer instead, which handles the absorption side properly.
Related Tools And Reading
- Thyroid Medication Timing Optimizer
- Optimal Thyroid Lab Test Calculator
- Levothyroxine Dose Adjustment Calculator
- T4 to T3 Ratio Calculator
- List of Prescription Medications That Cause Thyroid Problems
- High TSH and Normal T4: What Does It Mean?
Frequently asked questions
Far more than the two everyone hears about. Calcium and iron get mentioned at the pharmacy counter because they block absorption, and that is real, but absorption is only one of at least eight ways a drug can reach your thyroid.
Drugs can damage the gland itself, slow the conversion of T4 into the active T3, move your TSH up or down independently of your thyroid, change the carrier protein that transports your hormone, speed up how fast your liver processes it, or interfere with the blood test rather than the hormone. The checker above sorts your own list into those groups.
Yes, and it is common rather than rare. Thyroiditis is one of the most frequent immune-related side effects of the checkpoint inhibitors, the class that includes pembrolizumab, nivolumab, ipilimumab, atezolizumab and durvalumab. A meta-analysis of these regimens found meaningful rates of hypothyroidism across every combination studied [1].
The pattern is often a short overactive phase as the gland is damaged and releases stored hormone, followed by a permanent underactive thyroid. It usually needs treating for life. If you are on any of these and you feel exhausted, cold or foggy, that is worth testing rather than putting down to the cancer treatment in general.
Yes, and by two mechanisms at once. In a prospective evaluation, a substantial proportion of patients on sunitinib developed hypothyroidism [2]. The mechanism turned out to involve both induction of type 3 deiodinase activity, which destroys thyroid hormone faster, and regression of the capillaries feeding the gland [3].
That is why the checker flags the tyrosine kinase inhibitors under two headings rather than one. Sorafenib, lenvatinib, cabozantinib, axitinib, pazopanib and imatinib belong in the same conversation.
The most common explanation is that only TSH was checked. Several medications push TSH down without doing anything helpful for you. Glucocorticoids such as prednisone do it, and so do dopamine agonists, somatostatin analogues and metformin, where a meta-analysis confirmed a genuine reduction in TSH [4].
A second explanation is conversion. Steroids, propranolol, amiodarone and propylthiouracil can slow the step where T4 becomes T3. The result is a normal TSH, a normal free T4, a low free T3, and someone who feels awful. Neither pattern shows up unless free T3 is on the order.
Oral estrogen can, yes. It raises thyroid binding globulin, the protein that carries thyroid hormone through your blood. More of your hormone travels bound and less is free and usable, so a dose you were stable on may stop being enough.
The route matters more than the hormone. In a trial in hypothyroid women, oral estradiol raised binding globulin and several participants needed a levothyroxine increase, while transdermal estradiol alone changed nothing [5]. If you have a choice between a pill and a patch, that is worth raising.
Yes. Phenytoin, carbamazepine, oxcarbazepine and phenobarbital induce the liver enzymes that process thyroid hormone, so the same prescription delivers less of it. Thyroid status in patients on these drugs has been shown to need careful interpretation rather than a simple reference range check [6].
The practical version: if your TSH has drifted up on an unchanged dose since starting one of these, the drug is a likely reason. Rifampin and St John’s wort do the same thing.
Yes, and this one catches a lot of people. Biotin does nothing to your thyroid at all, but it interferes with the immunoassay most laboratories use to measure it. TSH reads falsely low while free T4 and free T3 read falsely high, which looks exactly like an overactive thyroid on paper [7].
Doses have been cut over this. Biotin turns up in hair, skin and nail formulas, B complex products and many multivitamins, often at doses far above anything you need. Stop it 2 to 4 days before any thyroid draw.
Amiodarone is one of the most disruptive drugs there is for the thyroid, and it can push in either direction. It carries an enormous iodine load, it blocks T4 to T3 conversion, and it can be directly toxic to thyroid tissue [8].
Anyone taking it needs thyroid monitoring as a matter of routine, not as a reaction to symptoms. The effects can also persist for months after stopping, because the drug is stored in fat.
Yes. Lithium concentrates in the thyroid and interferes with hormone release, and it is one of the better documented causes of drug-induced hypothyroidism [9]. It can also raise TSH and, less often, trigger thyroiditis.
It is not usually a reason to stop lithium. It is a reason to monitor thyroid function while taking it, and to treat the hypothyroidism if it appears.
It does, and the effect is autoimmune rather than chemical. Interferon alfa can trigger thyroiditis, and it is well described in patients treated for hepatitis C [10]. It can present as an overactive phase, an underactive one, or classic Hashimoto’s.
Interleukin-2 and alemtuzumab belong in the same group. If you have been treated with any of them, thyroid antibodies are worth checking alongside the standard panel.
The research
Scientific References
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3. Delivanis DA, Gustafson MP, Bornschlegl S, et al. Pembrolizumab-Induced Thyroiditis: Comprehensive Clinical Review and Insights Into Underlying Involved Mechanisms. J Clin Endocrinol Metab 2017;102(8):2770-2780. View on PubMed
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