Final answer:
In subclinical primary hypothyroidism, the thyroid gland produces insufficient amounts of T3 and T4 hormones. This leads to an increase in TSH while T3 and T4 levels can remain within normal limits, potentially causing a goiter but not necessarily severe symptoms.
Step-by-step explanation:
In subclinical primary hypothyroidism, the function of thyroid hormones (TFTs) is diminished, although not to the level of overt hypothyroidism. In this condition, the thyroid gland produces insufficient amounts of the hormones T3 (triiodothyronine) and T4 (thyroxine), which can lead to numerous symptoms like fatigue, weight gain, and cold sensitivity. However, these symptoms are often not as apparent in subclinical hypothyroidism compared to overt hypothyroidism.
The lack of thyroid hormones causes an increase in thyroid-stimulating hormone (TSH) secretion due to the body's attempt to stimulate the thyroid gland to produce more hormones. This increase in TSH, while T3 and T4 levels remain within or just below the normal range, is a hallmark of subclinical hypothyroidism. The thyroid may enlarge, leading to a goiter, because of continuous TSH stimulation, especially in cases of iodine deficiency where the thyroid cannot synthesize enough hormones despite the stimulation.