Cardiovascular System: When not enough thyroid hormones are produced, the heart rate slows down and circulation time is increased, which may make people feel tired and sluggish.
Musculature: Inadequate T3 and T4 levels contribute to muscle weakness, cramps, and stiffness.
Nervous System: Low thyroid levels in your brain may produce "brain fog."
The hypothalamus increases production of TRH when exposed to cold temperatures, which increases metabolism. The slowed metabolism and reduced circulation in those with hypothyroidism, along with constriction of blood vessels
close to the skin, may make the body feel cold.