There's nothing wrong with tracking CICO, per se. But, as noted (and implied by "proximal cause") the weight gain *is* indicative of a CI surplus. So, a proper accounting of CICO *will* demonstrate that. The question is, "what is the ultimate cause of the surplus?" In an unstable hormonal environment, the standard assumptions of a CICO can't be applied.

Under normal circumstances, a 29 year old male training for 70.3, and performing at a high level should have a hard time gaining significant weight, suddenly. When someone has a significant (>10%) and sudden (<6 months) change in weight there is cause for concern...especially, a young male training heavily. Further, the OP claims there's been no major change in diet, no increase in stress, and no uptick in Alcohol consumption.

So, its in that light that I made the recommendation....not simply because the OP gained a few pounds. I am semi-assuming (I did ask for clarification, pending a response) the OP has professional level healthcare...in which case, an Endocrinologist isn't really much more expensive than any other doctor.

All that said, an accurate accounting of nutritional consumption is likely to be useful to either the Doctor or a Nutritionist, regardless. So, I agree its still a good idea. Its the first thing a Nutritionist/RD will ask for...so, having it will short-circuit 3 weeks of delay.