Thyroid disease is more common in children with Down’s syndrome than in the general population. By the time they are adolescents around 15% of those with Down’s syndrome will have an underactive thyroid and the frequency continues to rise throughout adulthood.
A healthy thyroid plays a vital part in brain chemistry, so we should not be surprised that a thyroid disorder can cause unpredictable mood changes. For example those with an overactive thyroid (hyperthyroidism) can suddenly feel tense and anxious. They may experience panic attacks, impatience, be overactive and have an exaggerated sensitivity to noise.
Treatment of Primary Hypothyroidism and the L-T4/L-T3 debate
The 2015 position statement of the BTA
Why you may not feel instantly better despite treatment