Likely that one cat is depressed because the other cat is sick?

I have 2 cats...They are not siblings but they've been together for 12-13 years.  The female has been in rough shape for several months now.  She was diagnosed with hyperthyroidism about 2-3 months ago.  We're having problems getting her thyroid levels down with medication.  She's underweight, weak, and scraggly looking.

Around the same time her problems were getting more serious, my male cat started having frequent diarrhea.  He was at the vet earlier this year and the vet thought he probably had IBD.  I gave him medicine for a week or two and he seemed much better --until more recently.  It seems like right before I decide the diarrhea has gone on long enough and I'm about to make an appointment for the vet, he's better for a few days.  Now the diarrhea seems better but he's not too interested in eating and he seems depressed.  He seems pretty skinny too.  Perhaps this is due to his own health issues but I also wonder if it could be because the other kitty is so sick.  He's always been a sensitive kitty.  After the birth of my first child, he became a compulsive licker and licked himself half bald.

I think the primary thing is that I need to get him to vet again...it stinks that both are so sick right now.

Anyone have experience with a cat being depressed with another cat is sick?

Yes, I have.  I have an 18-1/2 year old cat who's had two of his companion cats die and has become very sick when they passed.  I was afraid I would lose him, too, this last time.

A vet visit sounds like a good idea to rule out physical causes.  But depression may be part of it.  If you search the word 'grieving' on this site, you'll see several threads on cats' grieving.  

Here's an article about it:  http://www.thecatsite.com/a/do-cats-mourn

It IS hard when there's more than one sick kitty in the house.     

Yes, cats do grieve.  It happened when I lost my 20 year old male a few years back and my, now, 20 year old male felt his loss deeply.

Yep, Mikki is grieving as we lost Minnie, her littermate and sister of 10 years a couple months ago. I'm so thankful she is eating well though. I'm so sorry about your kitty... its rough enough just worrying about one... let alone two.

Morgenne and May aren't littermates either, but they are the very best of friends. May was always a sickly kitten. The veterinarian wouldn't spay her until she was healthy, but unfortunately she developed pyometra (sp?) and we had no choice. After her surgery, she had to be confined to one room until she recovered as she needed a lot of help. Morgenne searched high and low for her, and even meowed (something she very rarely does). We caved in and let her in under supervision (lest she try to wrestle with her friend) and it put her at ease. Over the summer when I had to have Oliver put down, Morgenne repeated her search and meow, but this time I couldn't help her. It took her a long time to get back to normal.

Thank you, everyone.

It makes sense that since cats do grieve that they would respond when their pal is sick.  I don't hear them playing (or fighting) anymore or catch them grooming each other.