Jessica, Thyroid Stimulating Hormone is produced by the pituitary gland to stimulate the thyroid gland to produce thyroxine (T4) when it detects low circulating T3 hormone.
T4 is a pro-hormone which is converted to T3, the active hormone, in the thyroid gland and major organs.
Your TSH is low-normal but your FT4 is below range on both tests which means you are unlikely to be producing sufficient T3 and this is why you have hypothyroid symptoms.
You should ask your GP to consider secondary hypothyroidism (hypopituitarism) which is often caused by pituitary dysfunction. Your thyroid gland is probably healthy but isn't receiving sufficient stimulus to produce the required hormone. Treatment is Levothyroxine to replace the low thyroid hormone. TSH is not a reliable test in secondary hypothyroidism and FT4 and FT3 tests are the ones to rely on.
A pituitary function test should be done to check growth and sex hormone levels. When secondary hypothyroidism and pituitary dysfunction are suspected/confirmed patients are usually referred to an endocrinologist for investigation and management.
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