I assume the load cell has no amplifier. If it has typical ratings, a MCP3422 or MCP3424 would work well.
The LSB (Least Significant Bit) at full gain corresponds to about 2 microvolts.
This is an 18-bit ?? ADC with Differential Inputs. It has these features:
On-Board Programmable Gain Amplifier (PGA) with gains of 1, 2, 4 or 8.
On-Board Voltage Reference (VREF) Accuracy: 2.048V ± 0.05%.
I2C interface.
Here is an old post with some test code http://forums.adafruit.com/viewtopic.php?f=31&t=12269