Why do I have subclinical hypothyroidism?

Hello Doctor My name is Ashiya, and I have been dealing with subclinical hypothyroidism since I was 6 years old. It all started when I suddenly became very thin during my first grade. Concerned, my parents took me to a doctor who was already treating my mother for her subclinical hypothyroidism. After some blood work, the results showed elevated TSH levels at 10.5, while my T4 and T3 levels were normal. The doctor diagnosed me with hypothyroidism and prescribed thyroxine. Now, at 17, I want to understand more about hypothyroidism. Despite reading numerous articles and watching videos, I'm still unclear about the root causes of my subclinical hypothyroidism. I even don't have Hashimoto's thyroiditis. I've learned that deficiencies in selenium, zinc, copper, magnesium, and vitamin D can lead to subclinical hypothyroidism. My primary concern is whether this condition is permanent. I'm hesitant about taking a tablet every morning for the rest of my life . I would greatly appreciate your time to delve deeper into this condition.There's much to discuss, especially since my sister's TSH levels have recently elevated. We consulted a gynecologist [because my sister was not having her periods and the doctor got her thyroid test and found her TSH levels got elavated] and prescribed her 25 mcg of thyroxine, which I believe was inappropriate as her TSH levels were only at 9. Additionally, the doctor didn't test for antibodies. After 15 days of taking the tablets, my sister experienced throat pain and muscle pains. Now, her recent thyroid test showed a decrease to 8 without any thyroxine. We went to another doctor, who performed a TPO test and discovered that my sister does not have any antibodies. She's now focusing on her diet, incorporating brazil nuts for selenium, brown rice, and other foods rich in zinc, magnesium, and copper, as well as getting adequate sunlight for vitamin D. I'm hopeful that with your guidance, we can normalize her TSH levels and mine too without the need for lifelong medication. Could you please provide me with more information about this condition? Thank you. Sincerely, Ashiya.