Can hypothyroidism decrease intelligence/IQ?

Yes, if measured by timed tests. What hypothyroidism mainly does, "by definition", is slow down everything in the organism, including mental functions - so you actually still can think as well as ever, but it takes you longer. read more

So this is how it affected my intelligence, as it were. I’m still the same person in terms of my mental capabilities, but, they are slower at times, or I feel fatigued mentally, after a fairly short time. Hypothyroidism can cause mental illness and emotional illness too, and these in turn would certainly affect how we perceive and act in the world. read more

Adults who become hypothyroid, do NOT have a decrease in intelligence. Once they have adequate treatment, and every cell in their body has enough thyroid hormone(s), they are able to think and process information as before. read more

In adults, hypothyroidism does not leave permanent structural brain defects. But from experience, untreated hypothyroid gives a good impression of lower IQ. My particular symptoms of note in that regard: slow speech, dull facial expression, cognitive dysfunction. read more