Why would you just now be taking a script that was filled in 2014? Although medication may often be good past the expiration date listed on the bottle, it does lose potency over time, so you can't be guaranteed that it's as potent now as it was 5 yrs ago when the script was filled.
In addition, neither can be you be sure you still need the same dose, unless you've been taking the same medication at the same dosage all this time... which brings us back to the same question - why would you need to take medication that old?
Although you did not ask about this, I just wanted to make you are aware that there are scientific studies showing that achieving relief from hypothyroid symptoms typically requires Free T3 in the upper half of its range. In order to get Free T3 above mid-range, when taking T4 med, usually requires enough dosage to get Free T4 into the upper end of its range, or even above. This also usually suppresses TSH below range and causes most doctors to erroneously decide it represents hyperthyroidism, and decide to cut you med dosage. . That is wrong. In the treated state. a suppressed TSH represents hyperthyroidism only when you have hyper symptoms due to excessive levels of Free T4 and Free T3. An alternative many people find to work better for symptom relief is to add a source of T3 to their med, or to switch to desiccated, natural med like Armour Thyroid or NatureThroid, which contains both T4 and T3.
It would definitely be reduced efficacy so be careful. Levothyroxine is so cheap ifbyou canvget a new script