Levothyroxine drugs — Levothyroxine drugs like Synthroid, Levoxyl, and Tirosint are a synthetic version of thyroxine (T4) and are used to treat hypothyroidism.
Most experts agree that if you miss a dose of levothyroxine, you should take it as soon as you remember. If it’s been a full day since you forgot your pill, it’s still considered safe to take a double dose to “catch up.” Dr. Morris explains:
It generally works fine to take two levothyroxine tablets the day after missing one. This is because of the long half-life of T4 (seven days) and the relatively minor changes in levels that occur after taking the tablets — even two. In rare and difficult situations, where patients are unable to take medications daily or regularly (due to mental illness, for example), there have been studies that have demonstrated that it works reasonably to take seven tablets once per week if needed. (This is not our advice, but it does demonstrate the very long duration of action and half-life of this medication).
T3 Drugs — There are two types of T3 drugs: liothyronine and natural desiccated thyroid (NDT). Liothyronine drugs like Cytomel and compounded, time-released liothyronine are synthetic versions of triiodothyronine (T3) and are used — usually along with a levothyroxine drug — to treat hypothyroidism. NDT drugs like Nature-throid, Armour Thyroid, and WP Thyroid are natural drugs, derived from the dried thyroid glands of pigs, that contain natural forms of T4 and T3, and are also used to treat hypothyroidism.
If you miss a dose of a liothyronine or NDT drug, you can take your medication if you remember within a few hours. Dr. Morris does not, however, recommend taking a double dose if it’s been a full day since you missed your dose.
Doubling up is a bit more problematic because the pharmacology of T3 is very different than that of T4. The blood levels of T3 go up and down more quickly and dramatically after ingesting tablets. Thus, ingesting a double dose will exacerbate this and could, if done frequently, lead to some unfavorable symptoms in some individuals. If a patient missed a dose of a T3 medication, it is not advisable to “catch up” by taking a double dose.
The unfavorable symptoms Dr. Morris is talking include a rapid heart rate, heart palpitations, or nervousness, among others.
Antithyroid drugs — There are two types of antithyroid drugs used to treat hyperthyroidism. Methimazole — known by its brand name Tapazole — is the most commonly prescribed antithyroid drug. Propylthiouracil — a generic drug often known by its abbreviation PTU — is less commonly prescribed.
According to Dr. Morris, methimazole does not require taking it at precisely the same time daily. Doubling your dose a full day after missing a pill, however, is not recommended. Dr. Morris says:
If you forget to take your methimazole at your usual time in the morning, it’s safe to take it that same day in the afternoon or evening. Methimazole has a long half-life, but we don’t generally recommend doubling up after missing doses because of the risk of side effects. Patients should just resume taking the usual dose.
The guidelines are different for PTU. Dr. Morris says:
PTU generally requires divided dosing because of its short half-life. Thus, missing a dose here is just a missed dose. Patients shouldn’t “catch up” or take it later in the day unless they are taking PTU only once per day already.
Thyroid cancer patients — Remembering to take your thyroid medication is important for all thyroid patients, but missing doses is a particular concern for thyroid cancer patients on suppressive therapy. Patients on suppressive therapy receive a carefully calculated dosage of thyroid hormone replacement medication to keep TSH levels low — a proven way to help prevent recurrence of thyroid cancer.
According to Dr. Morris, if a patient on suppressive therapy to prevent thyroid cancer recurrence regularly forgets to take his or her levothyroxine, it can pose a risk. Says Dr. Morris:
The key is how often does the patient forget. If the patient misses even one dose per week, that is 14 percent of their weekly dose, which again because of the long half-life, reduces their daily dose by roughly a similar amount. This can be enough to allow the TSH to creep upwards and fall outside their target range for suppression. If the TSH is not suppressed, it can conceivably have a stimulatory effect on any tumors they may have. This is why we advise patients to “catch up” if they are sure they missed a tablet and to be as diligent as possible to get in all their doses.
Source: Email interview with Dr. John Morris, July 24, 2018
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You should know: The answer above provides general health information that is not intended to replace medical advice or treatment recommendations from a qualified healthcare professional.