Is it permissible for the husband to intentionally delay conception, making it conditional upon his wife losing weight, while the wife knows that she suffers from an underactive thyroid that affects the difficulty of her weight loss, and what has resulted from that in terms of psychological and physical harm to the wife, and a weakening in the marital relationship?

Your relationship with your husband before marriage is impermissible, and you must repent from it, for marital problems may be due to sins. Your husband's delay of procreation until you lose weight is not permissible, because it is an injustice and deprives both of your shared right to have children. Any harm caused during intercourse is not permissible in Sharia (definition), for the Prophet, peace and blessings be upon him, said: "There should be no harm or reciprocating harm." You should demand justice and kindness from your husband and remind him of Allah's covenant and the solemn promise between you, and the Prophet, peace and blessings be upon him, emphasized the rights of women. Let this be done gently, while increasing supplication for an improved situation.