My husband is 28 yrs, has hypothyroidism. He started medication, thyroxin, selenium+ brazil nuts. How long shud we weight before trying to conceive.?

No need to wait. You may start trying when both of you feel ready. You should consult your doctor and start taking pre-natal vitamins and establish a relationship with an obstetrician for your care during pregnancy.
Wish you both good health! - Have a diet rich in fresh vegetables, fruits, whole grains, milk and milk products, nuts, beans, legumes, lentils and small amounts of lean meats. Avoid saturated fats. Drink enough water daily, so that your urine is mostly colorless. Exercise at least 150 minutes/week and increase the intensity of exercise gradually. Do not use tobacco, alcohol, weed or street drugs in any form.