You need to ask your surgery for copies of any blood tests (of any kind) you've had in the last couple of years, complete with reference ranges. Copy them into a new post on here and ask for feedback. The ones that are of interest to us are thyroid functions tests (TFTs), any tests of nutrient levels, and a recent full blood count may be helpful if you have one. If your TFT results are mid-range then you are under-treated.
50mcg levo is only a starter dose, and the vast majority of hypothyroid people would need more than that.
In order to feel well, most hypos who are taking Levothyroxine need their TSH (Thyroid Stimulating Hormone) to be 1 or below, and Free T4 and Free T3 to be in the upper half of the range. Some need Free T4 and Free T3 to be in the upper third or upper quarter to feel well.
A slow heart rate and low blood pressure are common symptoms of hypothyroidism for a lot of sufferers. But eventually, if left under-treated, your blood pressure may rise substantially.
(Having said that, having a fast heart rate is not uncommon either.)
Going back to the issue of TFTs being mid-range...
Doctors assume that the distribution of TSH is a normal (symmetrical, bell-shaped) distribution i.e. most people would have a result in or near the middle of the reference range. But this isn't true. The actual distribution of TSH in healthy people who have no thyroid problems is highly skewed and looks like this :
Please read the text on the page - it is worth it.
The data that the graph is based on came from this paper :
The most important part of the paper (in my opinion) is the right hand half of Table 3 which shows the median TSH for each gender split into different age groups. So, for example, healthy women under 40 have a median TSH of 1.3. Healthy women in their 60s have a median TSH of 1.6. And yet the centre of the Reference Range for the test kit used in the study is 2.35. Almost 85% of the healthy population have a TSH reading below this value.
So if your doctor keeps your TSH to the middle of the range then he is keeping it higher than almost every healthy person in the population. And hypos (usually) need a lower TSH than healthy people to have a hope of feeling well.