last updated 3/27/2024
Even low-normal thyroid levels can raise the risk of depression or at least make depression harder to treat. So if you’re not getting better, check your thyroid hormone levels (if that wasn’t done before treatment started).
The numbers are backwards
First thing you have to learn about thyroid hormone levels: the numbers are backwards. Low means high and high means low. Huh?
Thyroid hormone itself can be measured, but for years we’ve been managing thyroid using a simpler test. It’s called TSH, thyroid stimulating hormone. TSH is the signal from your brain to your thyroid gland, saying “make more”. So when your thyroid hormone levels go down, your TSH goes up. Your brain is yelling at your thyroid gland, “make more!”.
And if you were to take oral thyroid hormone, raising your levels, your TSH would go down. The signal from your brain to your thyroid gland would become quieter.
Got it? High TSH means low thyroid hormone. Raising thyroid hormone, like with oral thyroid, means lowering TSH. Okay, next weird thing about thyroid hormone…
High-normal is sometimes not normal
Short version: the laboratory normal range for TSH goes from about 1.2 to about 4.2 (different labs use different standards). But even though a TSH of 4.1 is in the “normal range”, it’s pretty not-normal, actually. The most common TSH is around 1.5. Being up at the high-normal edge leaves some people more vulnerable to depression.
Bottom line: a TSH >2.5 is fine if you’re fine. But if you’re not, it could be part of the problem.
Another conclusion: if you have to take thyroid anyway, get to 1.0. Don’t stop at 3. There’s no greater risk near 1.0, and lower risk of mood problems, if you’ve had them.
For a long version of this story, see an essay I wrote for Psychiatric Times.
Thyroid as a treatment
For people with a partial response to an antidepressant, adding some thyroid hormone is a standard add-on treatment. Even for people whose TSH is in the “normal range” (probably better for those with TSH >2.5).
Thyroid hormone by itself, without the antidepressant, has not produced really great results. Added to something like lamotrigine? That’s not been studied much. But what if you’re considering low-dose lithium? If you’ve read about it, you know that it could lower your thyroid hormone levels. Here’s a sort of radical idea (I haven’t heard anyone else recommend it).
If you’re going to try lithium, you should always have your TSH checked first. If it’s higher than 2.5, especially if it’s over 3.0, you’re at greater risk of getting low thyroid hormone levels on lithium. You might end up having to add oral thyroid hormone. So maybe try adding thyroid hormone first? If your TSH gets down to around 1.0 and your mood is no better, then you could try lithium.
If that doesn’t work and now you’re ready to try lithium, it will be a little tricky figuring out if lithium then lowers your thyroid hormone levels. But if lithium works, great. There’s no hurry. Later you can try tapering off thyroid hormone while keeping the lithium. If your mood then worsens, or you get symptoms of hypothyroidism, then obviously you could put some thyroid hormone back in and get your TSH back down to around 1.0 again.
Yes, lots of blood tests to do all that. You can see why this is not a routine treatment strategy. But it’s extremely low in risk, compared to other options, because increasing your thyroid hormone — still within the normal range, just down near 1.0 — carries near-zero risk.