The invention provides compounds of formula (I) or a pharmaceutically acceptable ester, amide, solvate or salt thereof, including a salt of such an ester or amide, and a solvate of such an ester, amide or salt. The invention also provides the use of such compounds in the treatment or prophylaxis of a condition mediated by a thyroid receptor. Formula (I), wherein R.sup.1, R.sup.2, n, Y, Y', R.sup.3, R.sup.4, W and R.sup.5 are as defined in the specification.