Thyroid Function and the Lungs

When you can’t breathe, doing anything is tiring. Since low thyroid function slows down all physiology, it also slows down lung function. Doctors call this dyspnea, or shortness of breath and air hunger, because the lungs must work harder to breathe. So not only is thyroidhormone crucial in the development of the lungs, both lung tissue and surfactant layer, but it is also essential for respiratory muscle function and regulation when awake and sleeping. Newborns who neonatal distress syndrome have congenital hypothyroidism.

Many things are going wrong with respiratory function and thyroid dysregulation. You may have too much CO2 and carbon dioxide in your lungs, called hypercapnia. A reduction in lung capacity causes it, when you exhale, less air comes out, and more CO2 stays in because there is a reduction in the tension between oxygen from the heart and the oxygen in the lung alveoli. Respiratory variability, or regulation, is changed. Too much CO2 in tissues leads to acidosis, which is destructive and can cause insulin resistance, high blood sugar, kidney damage, and osteoporosis. This can cause a lack of pressure, similar to someone with pneumonia with a physical alveoli barrier caused by inflammation. This lack of pressure limits the diffusion of oxygen into the capillaries. It feels like something is blocking your lungs. You can breathe, but the oxygen cannot penetrate the arterial blood. You breathe harder, but less oxygen gets into the blood.

Hypoxia has the same effect as pneumonia. Someone who is hypothyroid can have hypoxia because there is less ventilatory drive. Hyperventilation, which is rapid or deep breathing often caused by anxiety or panic, also causes hypoxia because it depletes CO2 stores, and there is an automatic effect of hypoventilation to regain carbon dioxide and oxygen balance. You will undoubtedly feel anxious and panicky if you feel like that several times during the day. When stressed, adrenergic stimulation via the adrenal neurotransmitters—norepinephrine, also called adrenalin—ramps up the respiratory variability and ventilatory drive rhythm.

You usually don’t have to think to breathe; your body does it independently, without thinking about it. But there is something called blunted ventilatory drive, which occurs with asthma and chronic obstructive pulmonary disease. You breathe less without knowing it. The alarms to tell you you’re not getting enough oxygen don’t go off. In a large study, 34% of hypothyroid patients had reduced ventilatory drive. This will also lead to hypoxemia, too little oxygen in the blood. This causes hypercapnia, which is when there is too much CO2 in the blood. This leads to acidosis. Dyspnea, air hunger, or shortness of breath are pretty common. It doesn’t have to happen all the time. It usually occurs when you are under stress. Or at times of the day when you are tired. Remember, the adrenals, which send signals to all the organs, lungs, and heart to do their job, will falter when there is stress. With low thyroid hormone, those signals don’t get sent.

Low thyroid hormone also changes how the diaphragm works. The diaphragm’s muscle tone is crucial for healthy breathing. The diaphragm does breathing. It’s the muscle under your sternum, and its action creates negative pressure, so your lungs fill up with air, and then positive pressure, so the lungs empty. With low thyroid function, the diaphragm may not contract properly—the contraction is slower and less powerful. It can almost feel like a spasm in the muscle as if it won’t open to allow air in, so you cannot inhale. This may be a combination of muscle weakness caused by low T3 hormone and the disordered signal from the sympathetic nervous system—the adrenals — also caused by low T3 hormone, as hypothyroidism alters the balance between fast and slow twitch muscle fiber.

A weak diaphragm is the most common respiratory abnormality in hypothyroidism. The phrenic nerve enervates the diaphragm and may not send the right message. Several studies noted phrenic nerve paralysis in deceased persons with hypothyroidism. Hypothyroidism causes sleep apnea due to decreased ventilatory drive. Breathing is more challenging, and there is carbon dioxide retention. As breathing stops or slows down when a person goes to sleep, the oxygen level drops, and the carbon dioxide level is much higher than the level that triggers normal breathing. A high CO2 level can cause anxiety, daytime fatigue, headache, electrolyte imbalance, and high cortisol levels.

Finally, there can be water on the lungs or pleural effusions. Liquid accumulation is expected with low thyroid function. I’ve explained how liquid accumulates in the legs, tissue around the sacrum, and pericardial tissue. In this case, pleural effusion is due to structural changes in the capillary structure, all those tiny blood vessels in the alveoli, where the exchange of gas, which is the objective of breathing, occurs.

In severe hypothyroidism, there are fewer capillaries, their diameter is narrower, and they have increased permeability. Also, liquid accumulating in pericardial tissue can travel to the diaphragm. As a result, fluid accumulates, and the lymphatic system slows down. All this will contribute to water in the lungs.

Then there’s the small stuff. Breathing problems are common, with low thyroid function —from stuffy noses to recurring colds to discomfort or dryness in the throat. There is often an increase in nasal mucus and enlarged tonsils, which go back to the lymphatic system. So many hypothyroid people are using steroid inhalers and antihistamines. Their eyes itch, and they have hives and seasonal allergies. They may have sleep apnea machines and tape their mouths at night. This reduced lung function makes exercising hard. You want to do the right thing and exercise, but you cannot pull in enough air, your heart cannot beat fast enough, your muscles aren’t strong enough, or you are constantly tired because they are hypoxic. You try and try, push yourself, but just a few minutes of exercise feels like hours. Not enough oxygen comes in, so there will be abnormal levels of lactic acid. It all adds to less endurance and a massive drag of time, exercising, or just going up the stairs. It may not be why you are having difficulty breathing or exercising, but perhaps it bears investigating.