Nonnie It's my understanding that it affects the uptake, and means you need more replacement Thyroxine (Levothyroxine) than you might. I've had an under active thyroid for 20 years and it is quite difficult to get onto the correct replacement dosage. I was prescribed far too much by my previous GP (200 mcg) and had a "thyroid storm" because my thyroid went completely haywire and became over active and gave me atrial fibrillation, just as Willow mentions above. I was told I was on the verge of a stroke.
I am now on 75 mcg but it took 18 months for my body to adjust and I felt absolutely terrible, plus had to take beta blockers to get my heart under control.