Dorset92, In theory one thyroid lobe will pick up the slack and produce sufficient thyroid hormone. Over 20 years thyroid function can deteriorate whether it is the full gland or half. Pituitary gland doesn't *know* whether the thyroid gland is present and functioning, or whether half or all of it has been removed.
In primary hypothyroidism the thyroid gland dysfunctions. When pituitary detects low circulating thyroid hormone it increases TSH to stimulate the thyroid gland to produce more hormone. When sufficient hormone is detected pituitary reduces TSH. If the thyroid gland doesn't respond, or there is no thyroid gland, TSH levels will continue to rise until sufficient thyroid hormone replacement is provided.
In secondary hypothyroidism (pituitary dysfunction) and tertiary hypothyroidism (hypothalmic dysfunction) the thyroid gland is usually healthy but pituitary/hypothalmus dysfunction means the thyroid gland isn't receiving enough TSH stimulation to produce thyroid hormone and thyroid hormone replacement is required. The pituitary/hypothalmus dysfunction should be investigated in endocrinolgy as other hormones may also be deficient.
Your partner's FT4 is bottom of range, and with low-normal TSH, suggests possible secondary hypothyroidism (tertiary hypothyroidism is very rare), or isolated TSH deficiency. Her FT3 4.6 is surprisingly good considering her low FT4. It's low T3 which causes hypo symptoms. I think she should test FT4 and FT3 every 6 months and when FT4 is below range her GP should refer her to endocrinology if TSH hasn't risen.
It's worth your partner requesting ferritin, vitamin D, B12 and folate blood tests. Low/deficient levels can cause musculoskeletal pain, fatigue and low mood similar to hypothyroid symptoms.
I am not a medical professional and this information is not intended to be a substitute for medical guidance from your own doctor. Please check with your personal physician before applying any of these suggestions.