What causes lid lag in hypothyroidism?

Answer 1

Firstly lid lag is found in HYPERthyroidism, not hypo. The explanation for this is as follows:

If you (a normal person) takes an overdose of thyroxine, you would get lid lag, but not exophthalmos. This is because thyroxine is a beta adrenoceptor sensitiser. Thus the beta receptors in your heart are more active as adrenaline levels rise. Similarly, the beta receptors in levator palpabrae superioris (the muscles that open the eye) are more sensitive, and hold your eyes open. You will remember from Anatomy that the nerve supply to that muscle is half from the third nerve (occulomotor) and half from the sympathetic chain. This is the opposite of what happens when you cut the sympathetic chain (as in Horner's syndrome) when the eye will partly close.