Yes, an abnormally functioning thyroid can affect blood pressure.
In fact, both an underactive thyroid gland (hypothyroidism) and an overactive gland (hyperthyroidism) are associated with a rise in blood pressure.
Blood pressure and hypothyroidism
People with untreated hypothyroidism tend to have a higher diastolic reading with their blood pressure.
(Diastolic is the lower number in a blood pressure reading. The upper number is called systolic. A normal reading looks like this: 120/80. Your doctor would read this as "120 over 80.")
But often with hypothyroidism, both blood pressure numbers go up. Treating hypothyroidism with thyroid hormone replacement should bring the blood pressure down.
However, dont be disappointed if your blood pressure doesnt go all the way back to normal. High blood pressure is common. It's quite possible that you would have it even if you didn't have hypothyroidism.
By the way, hypothyroidism that isn't treated also can cause dramatic elevations in blood cholesterol levels. With thyroid replacement, the cholesterol levels will come down, too.
Blood pressure and hyperthyroidism
An overactive thyroid causes the bodys metabolism to speed up. Heart rate becomes faster and systolic blood pressure rises. However, diastolic pressure usually stays the same or goes down a little.
Treatment to decrease thyroid production by the thyroid gland will lead to a decrease in heart rate and a lowering of the systolic blood pressure.