Short answer: The Synthroid label has no missed-dose instructions. It describes one dose a day on an empty stomach, half an hour to an hour before breakfast. Levothyroxine (T4) has a half-life of 6 to 7 days in people with normal thyroid levels, so one missed tablet changes levels only a little. Ask your prescriber or pharmacist what to do.
| Situation | What the label says |
|---|---|
| Schedule | One dose a day on an empty stomach, one-half to one hour before breakfast (2.1) |
| Missed-dose rule | None in the label |
| Half-life of T4 | 6 to 7 days with normal thyroid levels (12.3, Table 10) |
| Full effect of a dose | May take 4 to 6 weeks (2.2) |
| Poor response at high doses | May point to missed doses, poor absorption or drug interactions (2.3) |
No. We checked the Synthroid prescribing information from AbbVie on DailyMed. It has no missed-dose section and no separate patient leaflet. Section 2.1 says only that Synthroid is taken as a single daily dose on an empty stomach, one-half to one hour before breakfast, and at least 4 hours apart from drugs such as iron, calcium supplements and antacids.
Generic levothyroxine and other brands have their own labels. This page covers the Synthroid label only.
Table 10 in section 12.3 gives a half-life for levothyroxine (T4) of 6 to 7 days in people with normal thyroid levels (3 to 4 days with an overactive thyroid, 9 to 10 days with an underactive one). Section 2.2 says the full effect of a dose may not be reached for 4 to 6 weeks. With numbers like these, levels drift slowly rather than drop in a day.
The only mention is in section 2.3: doses above 200 mcg a day are seldom needed, and a poor response to more than 300 mcg a day is rare and may point to missed doses, poor absorption, drug interactions or a mix of these. Regular missed doses can show up in thyroid blood tests, so they are worth telling your prescriber about.
This page summarizes FDA labeling. It is not personalized medical advice; confirm what to do with your prescriber or pharmacist.