If you have a swollen face, it may be caused by myxedema, one of the more common hypothyroidism symptoms.
Hypothyroidism causes an increased amount of water in your skin and a loosening of your collagen. This is due to a buildup of mucopolysaccharides, a type of amino sugar that attracts water and causes the collagen in your skin to separate. Your body's metabolism slows down when your thyroid gland is not working properly.
A slow metabolism creates less heat and drops your core body temperature. Your veins then constrict and become less elastic in response, leading to water and salt retention. This is why a swollen face can present itself as one of many hypothyroidism symptoms.
If you are experiencing swelling in your face, it is recommended to speak to your physician about testing your thyroid.
Harvard Health Publishing (2013). The lowdown on thyroid slowdown. Retrieved from https://www.health.harvard.edu/newsletter_article/the-lowdown-on-thyroid-slowdown
Kasumagic-Halilovic, E. (2014). Thyroid Disease and the Skin. Annals of Thyroid Research, 1 (2), 27-31.