The transcripts provided do not explicitly state what the "biggest thyroid problem" is, but they do discuss various aspects of thyroid function and common issues related to thyroid hormone regulation. The most commonly mentioned thyroid issue throughout the transcripts is hypothyroidism, which is a condition where the thyroid gland does not produce enough thyroid hormones. Hypothyroidism is discussed in relation to the treatment and regulation of thyroid hormones.

Here are some relevant timestamps and information related to hypothyroidism and thyroid hormone regulation:

- [00:03:34.000] - The active thyroid hormone, T3, is described as being responsible for various bodily functions such as metabolism, the condition of nails and hair, and bowel function. T4 is mentioned as the inactive thyroid hormone, which the thyroid gland produces mostly.
- [00:02:39.000] - The regulation of the thyroid gland is discussed, with TSH (Thyroid Stimulating Hormone) from the pituitary gland stimulating the production of T4 and T3 by the thyroid gland.
- [00:08:41.000] - Hypothyroidism is identified as more common than hyperthyroidism. The standard treatment for hypothyroidism is mentioned as giving synthetic T4.
- [00:09:17.000] - An example is provided of treating hypothyroidism with synthetic T4, where a patient with elevated TSH and classic symptoms of hypothyroidism may be given a specific dose of T4 to reduce TSH levels and alleviate symptoms.

While hypothyroidism seems to be a focal point in the transcripts, the information provided does not rank thyroid problems in terms of severity or prevalence, so it cannot be definitively stated that hypothyroidism is the "biggest" thyroid problem without additional context. However, based on the frequency and depth of discussion, it appears to be a significant concern when discussing thyroid health in the provided transcripts.