High altitude sickness or Acute Mountain Sickness (AMS) is a cluster of symptoms that affect people who ascend too rapidly to higher elevations, usually above 8000 feet (2500 meters). Many who plan a vacation to a high altitude ski area like Colorado end up with the typical symptoms of headache, insomnia, faster heart rate, and occasionally nausea, fatigue, dizziness, and loss of appetite. Hypoxia, or lack of oxygen, causes all of these symptoms, but what exactly happens that causes this?
Adrenal fatigue: low thyroid levels and low cortisol levels exacerbate high altitude sickness
Low oxygen triggers an increase in the thyroid D3 deiodinase enzyme that inactivates the thyroid hormones: T4 converts to reverse T3, and T3 is inactivated to T2. [1,2] So essentially, one becomes hypothyroid if they’re not already, and if they are already hypothyroid or undermedicated, they become worse. According to an article in a Denver, Colorado magazine, more than 20% of the visiting tourists suffer from high altitude sickness.  Maybe this reflects the number of undiagnosed or undermedicated hypothyroid patients? I certainly have an extremely difficult time at high altitudes and get a throbbing headache, faster heart rate, feel extremely parched unless I drink water constantly, and end up tossing and turning all night, when I’m usually a deep sleeper.
Glucocorticoids such as prednisolone, prednisone, or dexamethasone are used to treat altitude sickness. One study showed that both ACTH and plasma cortisol levels rose till day 8, after someone from sea level arrived by air to an altitude of 11,350 feet (3450 meters). Glucocorticoid treatment from day 1 on test subjects significantly lowered the severity of symptoms.  Hypothyroid patients frequently suffer from adrenal fatigue, where their cortisol levels have downregulated to match their lower thyroid levels. An inability of their adrenals to provide adequate cortisol would certainly result in a more severe state of high altitude sickness. The high altitude sickness symptoms of nausea, dizziness, and fatigue are also signs of low cortisol.
Thyroid hormones, norepinephrine, and cortisol were all elevated in a study of men who ascended to 14,270 feet (4350 meters).  Their thyroid and cortisol levels came up on their own as a natural response to the higher altitude. Anyone who is manually dosing their thyroid hormone will not have this natural response, but it suggests that a slight increase in T4 dosage might be helpful, since more than one study has shown a rise in T4 in all subjects at high altitude. In hindsight, the last time I was at high altitude, I did not sleep well at all and had a rapid heart rate. My Free T4 was at the very bottom of the range on desiccated thyroid, though my Free T3 was close to mid-range. I have since changed to a combination dose of desiccated plus T4, to balance both my T3 and T4 levels.
A low T3 syndrome was found in a similar study on men who climbed Mt. Everest and spent seven weeks at high altitude. They acclimatized at a base camp of 17,000 feet (5200 meters) for two months, before ascending to elevations of 24,600 feet (7500 meters) to 29,000 feet (8850 meters). None of them developed high altitude sickness. Bloodwork after returning to base camp showed increased Free T4 levels, but decreased Free T3 levels, which is usually a marker for high reverse T3. Their Free T3 levels dropped below the reference range, yet there was an average weight loss of 11 pounds (5 kg). There was no change in their TSH, DHEA-S, cortisol or ACTH from their levels seven weeks earlier at sea level.  The seven weeks may have given their bodies time to fully acclimate to the higher altitude. Interestingly, progesterone levels were higher, and progesterone is known to stimulate respiration.  Since none of these men developed severe high altitude sickness, it implies that the low T3 syndrome is the body’s way of handling lower oxygen levels, because lower T3 levels lower cellular oxygen requirements.
Reverse T3 was measured in another high altitude expedition, and reverse T3 levels rose during the trek, along with T4 levels and thyroxine binding globulin (TBG). Reverse T3 levels may have risen in response to physical exertion. 
Sleep quality was studied in two men shortly after they arrived at the South Pole (9300 feet or 2900 meters). There was complete loss of stage 3 and 4 sleep for all nights recorded. In addition, one man suffered from Acute Mountain Sickness and had a 100% increase in stage 1 and 50% decrease in rapid eye movement (REM) sleep.  Interestingly, hypothyroid patients also have significantly less stage 3 and 4 sleep, which are considered the deeper stages of sleep.
At high altitudes with lower oxygen levels, normal breathing and respiration rate are essential. Slow respiration is a hypothyroid symptom, and this may contribute to hypothyroid patients getting even less oxygen at higher altitudes, which would compound their symptoms. One study of hypothyroid patients found respiratory muscle weakness in both the inhalation and exhalation muscles and the weakness positively correlated with thyroid levels. In other words, the lower the thyroid levels, the weaker the respiratory muscles.  This muscle weakness is reversible if thyroid hormone is prescribed and thyroid levels are brought up.
High altitude will certainly be a problem for someone who is hypothyroid and possibly undermedicated, or undiagnosed. But knowing this, are there any measures that can be taken to reduce high altitude sickness symptoms, or must hypothyroid patients always choose low elevations to live, work, and vacation?
High altitude sickness treatments
Anyone on prescription thyroid hormone who still feels hypothyroid should have their thyroid levels tested, because their TSH may not reflect their thyroid levels. [TSH levels do not reflect thyroid levels]
Sleeping at an elevation below 6,500 feet or 2000 meters helps, because a lower sleeping altitude is one of the most important factors, along with rate of ascent, that determines whether someone develops high altitude sickness. Climb high, sleep low, is a rule that mountain climbers follow. 
Drinking lots of water to keep the body hydrated is important at high altitudes because of the dry air. Dehydration is a known cause of headaches, and for me, just drinking a lot of water keeps the headache away. Hypothyroid patients tend to be dehydrated even at sea level, because insufficient thyroid hormone often results in low aldosterone, a hormone involved in sodium, potassium and fluid balance.
Limiting physical activity in the first few days allows the body to acclimate to the higher altitude. If vacationing in a mountainous area, consider buying a half-day ski pass, or doing half-day hikes for the first few days. Check the elevation of any destination before booking the trip, and remember that 8000 feet (2500 meters) is when those who are susceptible usually become symptomatic. I had a coworker book a dream trip to the Alps with her daughter for her high school graduation gift. She (the mother) had no idea she could not tolerate the altitude till she was already there and had to return to the base city, while her daughter went ahead without her.
Sleeping at a lower elevation the first night before heading to the final, higher destination also helps the body acclimate. Denver, Colorado is already over 5,000 feet in elevation, but that is still less than the 8,000-13,000 feet found at the ski resorts. Look for other nearby towns with lower elevations for your lodging. Remember, one of the best ways to acclimate is to sleep low. Or pick a lower elevation ski resort altogether. Here’s a link to a table of ski resorts in the western US by elevation.
Alcohol and any drug that suppresses respiration (large doses of narcotic pain medications or sedatives like sleeping pills) should be avoided. Respiration normally slows when sleeping, is naturally slower in someone who is hypothyroid, and the addition of alcohol or drugs could result in dangerously low levels of oxygen while asleep.  People on thyroid internet forums have reported waking at night gasping for air when staying at high altitudes. Actually, when my thyroid levels were too low, I would also awake in the middle of the night gasping for air, and this was at sea level. This is called periodic breathing, where breathing slows and even stops for periods of 10-15 seconds.  It’s possible this is just another form of sleep apnea.  I would dream that I was underwater and running out of air, deep in a lake or the ocean, and would be swimming to the surface. I would awake with a huge gasp, right when my head would surface in the dream. My heart would be rapidly pounding when I woke, and a rapid heartbeat is one way the body compensates for low oxygen levels. I would also wake up in a sweat, and this could be from adrenaline that was pumped out to compensate for low cortisol levels, to keep blood sugar levels stable. I am happy to say that I have not had these dreams for a while, now that I’m on a higher dose of thyroid medication, when I used to have them every few days just a few years ago.
Diamox or acetazolamide is a drug that is sometimes prescribed to help high altitude sickness.  Caution is advised because this is a sulfa drug, and more than one patient on several thyroid internet forums has reported extreme reactions to sulfa drugs. I personally ended up at the ER in what seemed like anaphylactic shock when I was once prescribed a sulfa antibiotic.
Headache is usually the first symptom of high altitude sickness that most people experience and should not be ignored, because it can progress to high altitude pulmonary edema (HAPE) or high altitude cerebral edema (HACE) and left untreated, can be fatal. Swelling of the lungs and brain are the problem. Someone with high altitude pulmonary edema will have difficulty breathing and walking uphill. A dry cough and audible chest congestion are not good signs, nor are white lips and blue fingernails. Someone with high altitude cerebral edema will display loss of coordination (ataxia) and confusion. This can progress into a coma, and then death. With any of these symptoms, medical help should be sought, and it may be necessary to descend immediately to a lower altitude. 
There are many beautiful cities, parks, and ski resorts at high altitudes that are worth living, working, and/or vacationing in. It would be a shame to exclude visiting these areas because of a medical condition that is mostly correctable. Since high altitudes are more difficult to tolerate if someone is hypothyroid, they should certainly explore the possibility that they are undermedicated if they suffer from high altitude sickness, especially if TSH is used to determine their dose. Skiers may want to look into ski resorts with peak elevations below 8,000 feet. Here are a few reviews of lower elevation ski resorts.
Getting your thyroid tested
If you’d like to have your thyroid levels tested, please ask for these thyroid tests, and note where your levels are in the thyroid lab ranges compared to healthy people. If you do not ask for these specific tests, your doctor will most likely just run a TSH test, which sadly, does not catch many cases of hypothyroidism. [TSH levels do not reflect thyroid levels]
- Sabrina Diano, Tamas L. Horvath. Type 3 Deiodinase in Hypoxia: To Cool or to Kill? Cell Metabolism. Volume 7, Issue 5, 7 May 2008, Pages 363-364. http://www.sciencedirect.com/science/article/pii/S1550413108001204
- Yaluan Ma, Patricia Freitag, Jie Zhou, Bernhard Brüne, Stilla Frede, and Joachim Fandrey. Thyroid hormone induces erythropoietin gene expression through augmented accumulation of hypoxia-inducible factor-1. American Journal of Physiology – Regulatory Physiology, September 2004 vol. 287 no. 3 R600-R60. http://ajpregu.physiology.org/content/287/3/R600.short
- Lindsey B. Koehler and Natasha Gardner. Low On 02 – The ultimate guide to living at altitude: Slope Sick. 5280, The Denver Magazine. Oct 2009. http://www.5280.com/magazine/2009/10/low-02?page=0,2
- Basu, M., Sawhney, R. C., Kumar, S., Pal, K., Prasad, R. and Selvamurthy, W. (2002), Glucocorticoids as prophylaxis against acute mountain sickness. Clinical Endocrinology, 57: 761–767. doi: 10.1046/j.1365-2265.2002.01664.x https://www.thieme-connect.com/ejournals/abstract/hmr/doi/10.1055/s-2002-33260
- Jean-Paul Richalet, Murielle Letournel, and Jean-Claude Souberbielle. Effects of high-altitude hypoxia on the hormonal response to hypothalamic factors. American Journal of Physiology – Regulatory Physiol December 2010 vol. 299 no. 6 R1685-R1692. http://ajpregu.physiology.org/content/299/6/R1685.short
- Andrea Benso, Fabio Broglio, Gianluca Aimaretti, Barbara Lucatello, Fabio Lanfranco, Ezio Ghigo and Silvia Grottoli. Endocrine and metabolic responses to extreme altitude and physical exercise in climbers. Eur J Endocrinol December 1, 2007 157 733-740. http://www.eje.org/content/157/6/733.short
- A. D. Wright. Birmingham Medical Research Expeditionary Society 1977 Expediton: thyroid function and acute mountain sickenss. Postgrad Med J 1979;55:483-486 doi:10.1136/pgmj.55.645.483 http://pmj.bmj.com/content/55/645/483.abstract
- Albert T. Joern; Jay T. Shurley; Robert E. Brooks; Clarence A. Guenter; Chester M. Pierce. Short-Term Changes in Sleep Patterns on Arrival at the South Polar Plateau. Arch Intern Med. 1970;125(4):649-654. http://archinte.ama-assn.org/cgi/content/abstract/125/4/649
- Anthony Kales, Gunnar Heuser, Allan Jacobson, Joyce D. Kales, John Hanley,John R. Zweizig And Morris J. Paulson. All Night Sleep Studies in Hypothyroid Patients, Before and After Treatment. The Journal of Clinical Endocrinology & Metabolism. November 1, 1967 vol. 27 no. 11 1593-1599. http://jcem.endojournals.org/content/27/11/1593.short
- Siafakas NM, Salesiotou V, Filaditaki V, Tzanakis N, Thalassinos N, Bouros D. Respiratory muscle strength in hypothyroidism. Chest. 1992 Jul;102(1):189-94. http://www.ncbi.nlm.nih.gov/pubmed/1623751
- Thomas E. Dietz. Altitude Illness Clinical Guide For Physicians, May 2000. http://www.high-altitude-medicine.com/AMS-medical.html#periodic
- Altitude Hypoxia Explained, Altitude Research Center, Anschutz Medical Campus, University of Colorado Denver. http://www.altituderesearch.org/hypoxia/altitude-hypoxia-explained
- Tarja Saaresranta and Olli Polo. Hormones and Breathing. CHEST.December 2002. vol. 122 no. 6 2165-2182. http://www.ncbi.nlm.nih.gov/pubmed/12475861
- Andrew J Peacock. Oxygen at high altitude. BMJ. 1998 October 17;317(7165): 1063–1066. http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1114067/
- Taylor AT. High-altitude illnesses: Physiology, risk factors, prevention, and treatment. RMMJ 2011;2(1):e0022. doi:10.5041/RMMJ.10022 http://www.rmmj.org.il/userimages/41/1/PublishFiles/44Article.pdf