Synthroid+insulin - my bg's are out of control since starting synthroid - please share your experience!

Hi -

I've been taking care of my type1 diabetes pretty well these days - I got a dexcom and got my a1c down to 6.0. But... I recently found out about my hypothyroid problem and starting taking synthroid about a week ago, and since that day, my bg's have been out of control. I take humalog and lantus. It seems the insulin is MUCH slower acting (instead of taking my humalog about 15 min before eating, I need to take it an hour (or more!) earlier! (or otherwise I skyrocket to at least 300+!). I have also been crashing (I pretty much a yoyo all day). I figured that I would just need to tweek my dosage, but since taking synthroid, I can't seem to get it under control, so am thinking/hoping that maybe novalog doesn't interact so poorly with synthroid as humalog seems to be?

If you take synthroid, would you please let me know if this happened to you also, or if you do take humalog (or another quick acting insulin) and this was NOT a problem for you (maybe this is more of an issue w/ humalog than novalog)?


Oh, we had a discussion going on about this in the Hypothyroid Diabetics group… Thyroxine and high blood sugar. :slight_smile: All welcome to join.

I use Novolog with an insulin pump so my situation taking Synthroid doesn’t equate to yours, but I didn’t notice any difference in BG control when I started Synthroid. I think I would be making a call to my endocrinologist first thing Monday morning or talking to my pharmacist over the weekend to ask about that side effect if I were you though.

Western Washington

I do know that if we forget to give my niece her Synthroid (we give it on an empty stomach before bedtime), the next day her blood sugars will drop all day; so have surmised (maybe incorrectly for all, but for us) that you need more insulin as basal when you are taking Synthroid. She only takes a half pill but if we forget we have to reduce basals ten percent across the board.

Does she take novalog or humalog? Thanks

I’m really glad to hear that you didn’t notice any changes while taking novolog. I will definitely be talking to my doc on Monday and am hopeful now that maybe a change to novolog will help! - thanks for writing!

Fyi (or anyone else out there interested) - I was paroozing the internet last night and found a blog from someone that sounds to have had very similar swings while using R (regular) insulin and synthroid -

Darn - I actually read Michelle’s reply 1st (below) so I was hoping that the swings are more of a Humalog/synthroid issue than a Novolog/synthroid problem. I guess it’s different for everyone, so I’ll have to keep experimenting!

I’ll let you how taking Novalog compares to Humalog with syntrhoid (or maybe I’ll try the apidra - depending what the doctor suggests/gives me a prescription to try)…

thanks for the reply

I use Apidra & Humulin Regular. The change I noticed was needing more insulin across the board, 40-50% more. But, insulin didn’t work slower for me.

Yes yes yes!

It does get a little better though, once you’ve reached your ideal synthroid dosage, but the IR remains. I didn’t notice any change in action, but the amount was definitely a 40-50% increase, almost immediately. BG became much more variable as well.

What I find really curious though, is that my endo told me that the synthroid shouldn’t have “much” effect, but my CDE told me flat out “yes, you will need more insulin - probably a lot more”… so it’s hard to say how everyone reacts for sure. I have noticed a distinct shift in insulin needs every time my dosage has been changed.

I started Metformin at the end of January to combat the insulin resistance… it has helped some, though I wonder at this point if it isn’t really helping with some underlying issues I was having before, not just the increased IR. My DP is almost non-existant now though, and I don’t spike “as bad” after exercise. (mid-upper 100’s instead of 300+). It has also smoothed out my insulin requirements for my cycle… as well as cleared up the mild acne problems I’ve had pretty much all my life.

thank you. What type of insulin do you take and have you tried changing your insulin while on synthroid? If so, how did they compare?

For what’s worth, I have been taking Synthroid for 15-20 years and never noticed any insulin resistance related to it. I have taken regular, humilin, humalog and novolg during that time. I have taken both brand name Synthroid and the generic version.

I understand that when your thyroid hormone level is fluctuating, that this can affect your BGs. Maybe once your level evens out, the BGs will calm down?

Are you taking the generic? I had to switch back to the brand name because the generic was less predictable. My hormone levels would fluctuate and my symptoms would sort of “flare up.” The CDE told me that the generic version of Synthroid is not exactly the same as the brand name, and that the generic manufacturers are allowed something like a 10% variance in the strength of the pill they produce. So depending on the manufacturer that the pharmacy is selling that month and the batch, you can get inconsistent amounts of the hormone. I guess some people are more sensitive to this variance than others, so the generic works just fine for some.

I agree that a chat with the doctor is probably in order. I hope you can get this straightened out soon!

She used Novolog for years; recently switched to Apidra. Forgot again the other night and still a ten percent reduction for next 24 hours. She does use a lot of insulin for her body weight as basal but she went through puberty and just turned 13. I am sure her needs will go down once she is fully grown.

I take Armour natural thyroid replacement instead of synthroid. It contains T3 as well as T4 and it doesn’t affect my BG levels.

Yes, I’ve changed from Novolog to Apidra, but I was having an allergic reaction to the Novolog so while my insulin needs went down, it wasn’t related to anything else.

I too am a type I and 2 years ago got thyroid cancer. I am taking levathroid (sp???) but anyways it is very similar to synthroid. It might be worth asking your Endocrinologist. I assume you are seeing an endo? If not I would really suggest it. I must say it is really tough to keep it all balanced. I just have learned to kinda go with it and do the best I can. My A1c ‘s aren’t the best …7.1 But in light of my cancer I figure I am doing ok. I do test A LOT of times per day…like 8-10 x’'s because other wise I have no clue. I used to know just by the way I felt, but since the cancer I have lost the ability to tell so I test all the time. It is such a tough thing to balance…Sorry you are having such difficulties. If I can be of any help please feel free to message me. Take care

I have been on Synthroid for 15 years. I was diagnoses with t1 last year. This is how I take my Synthroid. I put it on the nightstand and when i wake up for a sip of water in the middle of the night I take the pill. Before being diabetic I would take it first thing in the morning and with nothing else. Why don’t you try taking it before bed. The important thing is to take it about the same time each day and with nothing in your stomach for about an hour. If you have any other questions about it please ask me. Also, it takes a full two weeks for the effects to occur in your body. Maybe the high glucose is due to something else…like the low levels of thyroid hormone you have. Just a thought!