Siobhan T. Pittock, M.B., B.Ch., Pediatric Endocrinology, Mayo Clinic: Hello, I'm Siobhan Pittock. I'm a pediatric endocrinologist at Mayo Clinic Children's Center and today I'd like to talk to you about hypothyroidism.

To have a better understanding of hypothyroidism, I think it's important that you understand what the thyroid gland does and how it's regulated. So thyroid gland is a gland that sits here in the neck, in front of the airway, and it's shaped like a butterfly. It makes a hormone called thyroid hormone or T4. T4 is what we call it for blood tests and the function of thyroid hormone or T4 is to regulate the speed of the cells of that the body is working. So if there's too much T4, that we call that hyperthyroidism. Everything in the body is sped up, so you have lots of energy. You have a shake potentially. You have fast heart rate. You have a lot of sweating. You lose weight easily and it's not very healthy.

On the other hand if you have not enough thyroid hormone, we call that hypothyroidism. Everything is slowed down and when things are slowed down, you feel very tired. Your heart rate is slow. You may feel colder than you should feel, want to wear extra jackets in the summertime. You often have dry skin or dry hair and you may have constipation. So it's important that the body really functions at the right speed, neither too fast nor too slow, and because that is such an important function, the body has the thyroid regulated not just by itself but by a higher body called the pituitary gland.

The pituitary gland sits in the brain, in the center between the eyes, just behind the eyes and in the center, and the pituitary gland regulates lots of glands around the body not just the thyroid gland. How it regulates things is that the pituitary gland is what tells the thyroid gland what to do and how it does this is by secreting a hormone called thyroid stimulating hormone or TSH for short. Under the influence of this TSH, the thyroid gland makes thyroid hormone. The pituitary gland is constantly aware of how much thyroid hormone there is in the body, and we call that a feedback loop. So the pituitary gland knows how much stimulation the thyroid gland needs by recognizing how much thyroid hormone there is in the system. So what happens in hypothyroidism, it can be caused by two separate events. Either your thyroid gland gets sick and that's the most common situation by far. The thyroid gland gets sick we call that primary hypothyroidism. If the thyroid gland is sick, it doesn't make enough thyroid hormone so we get a low T4 level. Because we have a low T4 level, the pituitary gland recognizes that and says, "Hey thyroid, wake up make more hormone" and increases the amount of stimulation it's giving. So in primary hypothyroidism, we have a low T4 because of a sick thyroid gland which results in the pituitary gland shouting louder at the thyroid and increasing the TSH, and that can be confusing for people because we talk about hypothyroidism, people know that it's underactive, and yet we talk about a high blood test. This is why. It's because it's a high TSH.

The second situation where we can get hypothyroidism is if your pituitary gland itself is sick. If your pituitary gland itself is sick, it isn't going to make the TSH, and so even a healthy thyroid gland can't respond because it's not being told what to do and so you have a low T4 level. That's much less common. We call that secondary hypothyroidism.

Whether you have primary hypothyroidism or secondary hypothyroidism, the treatment is the same. What we do as physicians as providers is we give you the replacement. This is what you need the T4 and we have a really good chemical substitute for that. It's called levothyroxine and we give you the levothyroxine and it's a once daily pill and then we figure out how you're doing based on your symptoms but also based on measuring these two blood tests. We can tell that very easily if we have given you the right amount of medication by checking those blood tests. It doesn't take very frequent monitoring. We check it more frequently in children because they're growing and their doses need to be adjusted more frequently, and then in adulthood, generally people only need to have their thyroid blood tests checked once a year. For the most part, primary care providers, whether they be pediatricians or family doctors, are comfortable looking after hypothyroidism once the initial diagnosis has been made and once people have started on therapy.

Oct. 28, 2022