Thyroid disease

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An overactive thyroid, also known as hyperthyroidism or thyrotoxicosis, is where the thyroid gland produces too much of the thyroid hormones.

The thyroid is a small butterfly-shaped gland in the neck, just in front of the windpipe (trachea). It produces hormones that affect things such as your heart rate and body temperature.

Having too much of these hormones can cause unpleasant and potentially serious problems that may need treatment.

An overactive thyroid can affect anyone, but it's about 10 times more common in women than men, and typically happens between 20 and 40 years of age.

https://www.nhs.uk/conditions/overactive-thyroid-hyperthyroidism/

An underactive thyroid (hypothyroidism) is when your thyroid gland does not produce enough of the hormone thyroxine (also called T4).

Most cases of an underactive thyroid are caused by the immune system which usually fights infection, attacks the thyroid gland, and damaging it, or due to thyroid surgery for overactive thyroid problem or thyroid cancer. 

A condition called Hashimoto's disease is the most common type of autoimmune reaction that causes an underactive thyroid.

It's not clear what causes Hashimoto's disease, but it runs in families. It's also common in people with another immune system disorder, such as type 1 diabetes and vitiligo.

https://www.nhs.uk/conditions/underactive-thyroid-hypothyroidism/causes/