Millions of people reach the point in life in which they need thyroid replacement. Some develop Hashimoto's thyroiditis, the auto-immune form of thyroid deficiency. But, many become hypothyroid without developing the antibodies for Hashimoto's.  It's like their thyroid gland just conks out, perhaps from aging, but also, perhaps, because of anti-thyroid substances that occur widely in foods.  Thyroid hormone is the most prescribed drug in America. 

Thyroid hormone occurs in two forms: T3, which is the active hormone, and T4, which is the storage form of the hormone. The number refer to the number of iodine units it has. So, the body has to split off one of the iodines to make the hormone active.

Most medical doctors prescribe the synthetic form of T4 which is known as Levothyroxine or Synthroid. But, the problem is that many people don't convert it into T3 very well. They don't get any bump in T3 at all. There is a synthetic form of T3, but it's rather dicey; people tend to have reactions to it; it requires closer monitoring.  Hence, many doctors just settle for the Synthroid.  

However, there is also the natural dessicated thyroid, which is derived from baby pigs,  which has both T3 and T4, and it is generally easy to take without difficulty. The ratio of T3 to T4 is weighted slightly higher to T3 in a pig than a human. But, it doesn't seem to matter, clinically, in most people. To my mind, the only thing Synthetic T4 can be counted on to do is push down the TSH. Since T3 is the active hormone, that's what matters most. And for people who don't convert synthetic T4 into T3, they really need a different and better solution. Dessicated thyroid may be that solution. It really may be the best option for a great many people, even though it's animal-derived. I know that many alternative/complementary doctors prefer to prescribe the natural dessicated thyroid, and even many regular doctors do. Hilary Clinton's doctor prescribed dessicated thyroid to her. 

I am not a medical doctor, so I don't prescribe thyroid to anyone. However, when it's an issue for someone, I will usually refer that person to a wholistic M.D. who prescribes dessicated thyroid.    

As far as trying to rehabilitate the thyroid through nutrition and supplements, such as iodine, tyrosine, etc., in a borderline case, you can try it. But, when a person is clearly and profoundly hypothyroid, I don't think it's wise to delay giving them thyroid replacement. In fact, I think it is very unwise. It is a major strain on the body to not have enough thyroid. It's not something to trifle with.

And again, it affects millions. For millions, hypothryoidism is inevitable. And that's why I think that routine blood testing of people over 50 should always include a thyroid panel. 

And that is my take on low thryoid.