Rarely do we see signs of deficiency in adult animals. We are made aware of a need when kids develop a goiter ( swollen thyroid glands) or aborted kids who are delivered with enlarge glands and often hairless.
Thyroid hormones are essential for growth and development of the fetus, particularly the brain, lungs, heart and hair follicles, and contribute especially to survival of the newborn kid.
If you suspect you have a deficiency in your herd, you can test by painting iodine on the tail web ( skin on the under side of tail). If the tail absorbs the iodine within 24 hours then you have an issue. You should continue to paint iodine on the tail web until it no longer absorbs. This will help immedient need. You will also need to add iodine to their diet by adding Kelp or iodized salt out daily.
Keep in mind that certain plants, such as those in the Brassicas family can interfere with iodine uptake by the thyroid gland. Plants such as mustard family like, cabbage,Broccoli and turnips. If goats have access to these plants you will need to remove that access