Hello, dear readers! Welcome to the monthly letters column. You have been sending us some very interesting mail, so we will dive right in.

-- We recently wrote about osteonecrosis, a condition in which a decrease or loss of blood supply causes bone tissue to die. It occurs most often in the hip, but it is also seen in the knee, ankle, wrist and shoulder. A reader in Ohio has asked for more information. "It would be helpful to learn what can cause that lack of blood flow in osteonecrosis," she wrote. "I can imagine, without proof, some initiating causes could include lack of exercise, extreme atherosclerosis, years of smoking and trauma." Yes, you are on the right track. Each of the examples you named can adversely affect circulation. However, lack of exercise, extreme atherosclerosis and smoking would be more accurately described as contributing causes, not initiating causes. Trauma is the most common reason that bone tissue loses blood flow. Trauma can stop or gradually interfere with the vessels that nourish the bone. This can be from an accident that breaks the bone or from gradual wear and tear, as occurs with overuse or arthritis.

-- We have written in the past about lion's mane mushroom, a fascinating fungus with a history of use in traditional Chinese medicine. It has been used to support digestive health and general vitality and, in some traditions, to improve cognitive function. We heard from a reader who wanted to know about possible adverse drug interactions. "Does lion's mane mushroom powder interfere with blood pressure medications or synthetic thyroid hormones?" they asked. While studies into lion's mane mushroom are now underway, their primary focus is on cognition. Data on potential interactions with prescription medications is quite limited. For now, it's impossible to say whether lion's mane would interfere with these medications. That said, these medications support crucial areas of physical health. It is important to explore any use of the mushroom with your health care provider.

-- A column about the cognitive and mental health benefits of music, both for people who are healthy and those who have cognitive decline, continues to generate mail. A note from a California poet was an unexpected surprise. "Poets have known, and have recently discovered again, that the effects of speech carry more deeply into the psyche of audiences when accompanied by music," he wrote. "I have found this to be true in two decades of performance, though I had no scientific explanation for it." Your observations, and your life's work, align with this growing body of research. It suggests that music can influence attention, emotion and memory. This may be why spoken language can resonate just as deeply.

Thank you, as always, to everyone who took the time to write. You can address your thoughts, comments and questions to us at askthedoctors@mednet.ucla.edu. Let us know where you are writing from. We read all of your letters and look forward to hearing from more of you.

(Send your questions to askthedoctors@mednet.ucla.edu, or write: Ask the Doctors, c/o UCLA Health Sciences Media Relations, 10960 Wilshire Blvd., Suite 1955, Los Angeles, CA, 90024. Owing to the volume of mail, personal replies cannot be provided.)

Dear Doctors: A friend who was diagnosed with Hashimoto's disease is now fearful of getting thyroid cancer, which happened to another family member. A social media site advocates a rigid diet, such as no gluten, dairy, tofu or tomatoes. How strong is scientific evidence for these restrictions?

Dear Reader: Hashimoto's disease is a chronic autoimmune disorder that affects the thyroid gland. Located just below the Adam's apple and above the breastbone, the adult thyroid measures about two inches across. It has two main lobes and is shaped like a bow tie or a butterfly. The thyroid produces the hormones our bodies use to regulate metabolic rate. The gland plays a crucial role in managing heart rate and body temperature. It's also involved in brain development, bone health and weight management.

In Hashimoto's disease, the immune system mistakenly attacks the cells and tissues in the thyroid that produce these hormones. This reduces thyroid function over time. There are many possible symptoms. They can include fatigue, dry skin and muscle weakness. Other symptoms are facial puffiness, brittle nails, constipation, hair loss and a swollen tongue. Hashimoto's can also make people feel easily chilled, sleepier than usual or depressed, and it can even change menstrual intensity and rhythms. A variety of blood tests can diagnose the disorder. Treatment depends on how bad the condition is. Options range from watchful waiting to hormone replacement therapy.

Hashimoto's disease is not considered to be a precancerous condition. That said, some studies have found a higher rate of papillary thyroid cancer among people who have Hashimoto's. The reasons for this are not yet clear. It's possible the chronic inflammation, a common factor in autoimmune diseases, plays a role. Celiac disease is a condition that occurs more often in people with Hashimoto's than in people without. Celiac disease is also an autoimmune disease. It's triggered by ingesting gluten, a protein found in wheat, barley and rye. In celiac disease, gluten causes the immune system to attack the lining of the small intestine.

That brings us to the dietary restrictions your friend has been reading about on social media. The warning about gluten is because celiac disease and Hashimoto's disease often occur together. The warning for dairy products is because lactose intolerance symptoms can mirror and worsen the symptoms of celiac disease. Avoiding tofu is based on evidence that some compounds in soy products can reduce absorption of levothyroxine, a synthetic hormone used to treat some forms of thyroid disease. The stance on tomatoes reflects a belief, still under debate, that nightshade vegetables can cause inflammation.

While our overview can be helpful to your friend, this is not medical or dietary advice. Instead, we hope that they will use the information to craft questions to ask their own medical team. Each case of every disease is unique to the patient and needs guidance specific to their needs and circumstances.

(Send your questions to askthedoctors@mednet.ucla.edu, or write: Ask the Doctors, c/o UCLA Health Sciences Media Relations, 10960 Wilshire Blvd., Suite 1955, Los Angeles, CA, 90024. Owing to the volume of mail, personal replies cannot be provided.)

Dear Doctors: I have always liked how yawning feels and wondered what it's for. I just read there is new information about how yawning affects your brain. Do you know anything about that? Also, do we know yet why yawning is so contagious? I've even been able to get my dog to yawn.

Dear Reader: Your questions about yawning echo across millennia. A physical reflex still shrouded in mystery, the search for an answer to why we yawn dates back at least to the ancient Greeks. Aristotle and Hippocrates favored a theory of ventilation. Less scientific notions have leaned into the mystical. Some have guessed soul slippage, a leaching of life force or spirits entering or leaving the body. What we do know for sure is that virtually all primates yawn. The behavior has also been observed in birds and even some fish. And you're right that it's contagious. It would be surprising if a portion of readers hadn't already stifled, or given in to, a yawn while reading this column.

Now, the results of a small study suggest the ancient Greeks weren't that far off the mark. Researchers in Australia have found that yawning can affect the movement of cerebrospinal fluid in the brain. Cerebrospinal fluid, or CSF, is a clear liquid produced in the brain. It helps to cushion and protect this critical organ. CSF also carries nutrients and removes waste.

In their study, scientists performed MRI scans of the brains of 22 healthy volunteers as they breathed normally, took deep breaths, yawned and stifled a yawn. Because deep breathing and yawning share similar physical actions, the researchers expected to see similar MRI results. However, the scans showed that during a yawn, cerebrospinal fluid was often transported away from the brain. This was the opposite of what happened during a deep breath.

This led the researchers to conclude that, rather than a variant of deep breathing, yawning is a distinct maneuver that reorients the flow of CSF. As for why that is metabolically advantageous, the answer is not yet clear. Theories include the idea that the movement of CSF plays a role in cooling the brain, aids in the removal of metabolic waste or amps up the alertness that is critical to scanning for danger.

While the findings in this study took a surprising turn, it is important to note the small sample size. In addition, the movement of CSF during a yawn was not observed 100% of the time. Also, the effect was observed more often in women than in men. In an interesting side note, each person yawned the same way and in the same pattern. Also, each study participant's yawn sequence was unique. Sadly, no light was shed on why yawns are so contagious.

As with all preliminary research, larger and repeated studies are needed to affirm the results. However, even without a conclusive outcome, the study bolsters the existing idea that yawning likely serves a specific purpose. It's something to consider next time you give in to a yawn, or to the urge to get your dog to yawn along with you.

(Send your questions to askthedoctors@mednet.ucla.edu, or write: Ask the Doctors, c/o UCLA Health Sciences Media Relations, 10960 Wilshire Blvd., Suite 1955, Los Angeles, CA, 90024. Owing to the volume of mail, personal replies cannot be provided.)

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