Thyroid hormone is essential for normal brain development. Cretinism (old term for a state of mental and physical retardation) occurs if neonatal (infant) hypothyroidism is untreated.
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.