Dear Doctor: My mother and father have hypothyroidism, as do my two younger sisters. Now I've been experiencing symptoms that would indicate thyroid problems. My doctor checked my T3 and TSH levels, but did not check T4, and said I did not need medication. Shouldn't she also have checked my T4?

Dear Reader: Your question suggests that you've done some homework. Good job! As you know, hypothyroidism is the term used when the thyroid gland doesn't produce enough thyroid hormone. The condition is quite common, occurring in about 4.6 percent of the population. Further, it's five to eight times higher in women than men.

The most common cause of hypothyroidism is Hashimoto's thyroiditis, a slow autoimmune attack of the thyroid gland that leads to decreased production of thyroid hormone. Genetic susceptibility can indeed play a role in an autoimmune attack of the thyroid, meaning having one family member with hypothyroidism increases an individual's risk of hypothyroidism. Because you have multiple family members with hypothyroidism, I can understand your concern about developing this disease.

Measuring the level of TSH, thyroid-stimulating hormone, is a very good test for hypothyroidism. TSH, which is produced by the pituitary gland, stimulates the thyroid to produced thyroid hormone. Alternatively, in a feedback mechanism, the thyroid hormones T3 and T4 suppress secretion of TSH. So, if your thyroid hormone levels are high, your TSH level should be low, and if your thyroid hormone levels are low, your TSH level should be high.

In other words -- because of the feedback mechanism -- when you have hypothyroidism, your TSH should be higher than normal. Because the feedback mechanism is slow to change the level of TSH, the TSH level gives an indication of your overall thyroid status over many weeks. One caveat with TSH is that it increases as you get older, meaning that people over the age of 80 can have higher TSH levels, but not be considered hypothyroid. Because your TSH is normal, I would think that hypothyroidism is of low likelihood.

But let's take a closer look at T4 and T3. The thyroid produces these hormones in response to TSH; 80 percent of the hormones produced by the thyroid is T4, and only 20 percent is T3. The latter is a much more active thyroid hormone, with three to five times the effect of T4 and a much shorter half-life. That means its levels vary more than T4 levels from one day to the next. Also, T3 drops substantially with illness. It's true that T4 can be a more precise test for thyroid hormone status, but in your case I don't think there is additional value in checking T4 because both your TSH and T3 levels are normal.

That said, while your thyroid function on these tests are currently within range, your doctor should continue to monitor them because your family history puts you at increased risk of hypothyroidism. Be aware, however, that severe illness can affect the levels of TSH, T4 and T3, so thyroid function assessments are not accurate under those circumstances. At those times, checking a reverse T3 may be helpful.

Finally, keep in mind that the symptoms of hypothyroidism (fatigue, constipation, cold intolerance, weight gain and dry skin) can be caused by many other conditions. It's certainly smart to have your thyroid checked when you have these symptoms, but if your thyroid is normal, you should probably consider a broader evaluation.

(Send your questions to askthedoctors@mednet.ucla.edu, or write: Ask the Doctors, c/o Media Relations, UCLA Health, 924 Westwood Blvd., Suite 350, Los Angeles, CA, 90095. Owing to the volume of mail, personal replies cannot be provided.)

Dear Doctor: My best friend, who never smoked, died of lung cancer last spring. My father, who has been a smoker since high school, is fine. What puts you at greater risk of getting cancer -- your genes, your habits, the environment, or is it just random chance?

Dear Reader: Your question is familiar ground for anyone whose life has been touched by cancer. It's not so much a wish to assign blame as it is our minds seeking a toehold of logic in the face of catastrophic news. If we know how or why something happened, then life events can be less arbitrary, and thus more manageable.

When it comes to lung cancer, decades of research have substantiated the correlation between the use of tobacco products and lung disease. No, not everyone who smokes goes on to develop lung cancer. But those who smoke, like your father, do increase their risk. However, two recent studies into your question have added some nuance -- and more than a little controversy -- to the discussion.

Please join (and excuse) us as we wonk out a bit.

Last winter, the authors of a paper published in the journal Science returned with a more targeted take. The original paper posed the same questions you're asking. The conclusions led many readers to understand that the authors were saying plain old bad luck is responsible for a majority of cancers. Needless to say, the pushback was fierce.

The newest paper, published by the same authors last March, still comes down on the side of random chance. However, with a larger sample size and expanded explanations and arguments, it also backs many of the ideas behind disease prevention.

The authors wanted an answer to why cancer arises from some tissues in our bodies a million times more frequently than it does in others. They focused on the fact that, in specific tissues that give rise to 31 types of cancer, stem cells divided far more often. Using a mathematical model, they concluded that amid that flurry of DNA replication, errors were occurring.

The new study arrives at the same conclusion: A majority (66 percent) of cancerous mutations result from random chance. However, this time the authors looped in how this relates to environmental factors and genetic predispositions.

Certain behaviors, like smoking, as well as environmental factors and genetics, do cause an increase in mutations. But just where in any individual's DNA these mutations will occur is unpredictable. In your late friend's case, mutations occurred in tissues that gave rise to lung cancer. Your dad's smoking may well have caused mutations as well, but thus far they have not been located in susceptible tissues.

This isn't a blank check to go off the rails. The study concluded that at least three separate gene mutations are needed to move a tissue from health to disease. Maybe one or two mutations will happen due to random chance. Why add to the odds of another mutation by smoking, eating poorly or not getting enough exercise? Bottom line -- when you take part in high-risk behaviors, you're loading the genetic dice.

(Send your questions to askthedoctors@mednet.ucla.edu, or write: Ask the Doctors, c/o Media Relations, UCLA Health, 924 Westwood Blvd., Suite 350, Los Angeles, CA, 90095. Owing to the volume of mail, personal replies cannot be provided.)

Dear Doctor: My 83-year-old mother has advanced osteoporosis. She and I recently read several articles touting the benefits of vitamin K2. Does it help strengthen bones? If so, what is the recommended daily dose? Are there side effects?

Dear Reader: Vitamin K is necessary for the coagulation of blood. There are two main forms, appropriately named vitamin K1 (phylloquinone) and K2 (menaquinone). The major source of K1 is green vegetables, while K2 is produced by the bacteria in the intestine. K2 also can be obtained from fermented soy, cheese and curds.

Vitamin K helps the hormone osteocalcin create more structurally sound bone in a process called carboxylation. When vitamin K levels are low, the carboxylate form of osteocalcin is also low, leading to more fragile bone. Vitamin K also increases the activity of cells involved in bone formation and decreases the activity of cells involved in bone destruction.

Low vitamin K consumption and decreased blood levels of vitamin K are each associated with an increased risk of hip fractures in men and women, so one could assume that taking supplements would naturally help decrease the rate of hip fractures. The evidence, however, isn't completely convincing.

A 2006 analysis of seven Japanese studies looked at the K2 supplement menaquinone-4. Six of the trials assessed the impact of 45 milligrams of menaquinone-4 in women over the age of 55. The trials lasted 12 to 24 months. The supplement led to a decrease in overall hip fractures by 6 percent, spinal fractures by 13 percent and all other fractures by 9 percent. The authors also showed improvement in bone density with the use of either vitamin K1 or K2.

A 2015 combined analysis of 19 randomized controlled trials similarly analyzed the impact of K2 supplements. Ten of the studies focused on people with osteoporosis, and the researchers found, as expected, an increase in the carboxylate form of osteocalcin among those taking K2. In women without osteoporosis, there was no difference in bone density among those who took K2. Women with osteoporosis showed an improvement of spinal bone density, but no change in hip bone density. (Note that, although this was a combined analysis, two-thirds of the patients came from one study in Japan.) Lastly, a 2016 Japanese study found improvement in bone density when vitamin K2 was combined with a bisphosphonate medication.

Few studies outside of Japan have assessed the impact of K2 supplements, so it's difficult to say whether supplementation might have the same impact for Americans. The typical Japanese diet is very different from the typical American diet, so factors other than K supplementation may play a role in fracture risk and bone density -- or vitamin K may be more necessary in a Japanese diet.

A 2009 study in the United States enrolled 381 postmenopausal women with low bone density to take either vitamin K1 (1 milligram), vitamin K2 (menaquinone-4 at 45 milligrams) or a placebo for 1 year. The authors found no difference in bone density or bone markers between the three groups, except that the carboxylate osteocalcin was higher in the groups that took either form of vitamin K.

That said, vitamin K2 supplements have been linked to greater amounts of nausea and abdominal pain compared with placebo. Of special note, vitamin K should not be taken with the blood thinner warfarin because it counters the effect of the drug.

Although the devastating impact of hip fractures underscores the need for improved bone-strengthening options, we need randomized trials in the U.S. and Europe evaluating K2 supplements before we can make broader conclusions about their benefit.

(Send your questions to askthedoctors@mednet.ucla.edu, or write: Ask the Doctors, c/o Media Relations, UCLA Health, 924 Westwood Blvd., Suite 350, Los Angeles, CA, 90095. Owing to the volume of mail, personal replies cannot be provided.)