Donna, if your GP suspects secondary hypothyroidism (hypopituitarism) then there is no point in TSH testing as it will always give a low result even though you may be very hypothyroid from lack of stimulus to the thyroid gland. FT4 and FT3 are the only tests valid in secondary hypothyroidism in order to measure how much thyroid hormone is circulating and your GP should write secondary hypothyroidism or hypopituitarism on the bloods order to make sure the lab does them.
Has your GP tested for thyroid peroxidase and thyroglobulin antibodies to rule out autoimmune thyroid disease (Hashimoto's)? Having one autoimmune disease does predispose you to others and steroids can affect thyroid function. If pituitary dysfunction is causing your hypothyroidism it's likely your thyroid gland is healthy but not receiving signals to produce hormone.
If FT4 and FT3 are low or below range your Levothyroxine dose will need to be increased every 6/8 weeks (usually in 25mcg increments) until they are in normal range. It takes 7/10 days for thyroxine to start working in your system before you can expect any improvement in symptoms but it may be some months after you are on an optimal and stable dose before you feel well.
Take your Levothyroxine with a full glass of water on an empty stomach one hour before or two hours after food and drink, two hours away from other medication and supplements and four hours away from vitamin D3, calcium, iron and oestrogen.
Ask your GP to test ferritin, vitamin D, B12 and folate as hypothyroid patients are often deficient/low and these deficiencies can cause musculoskeletal pain, fatigue and low mood similar to hypothyroid symptoms. Post your results with the lab ref ranges (the figures in brackets after your results) in a new question and members will advise whether supplementation is required.