It isn't a perfect match, but if it's possible the intervening half century has blurred your memory of the story a bit I'd like to suggest the novella Wings of the Lightning Land by Frederik Pohl (writing as James MacCreigh). It was originally written in 1941, but it was collected in the anthology Science Fiction: The Great Years in 1973 and that's probably where you read it.
The captain is Thorssen, only he isn't a captain. The iodine deficiency has become a major problem and the protagonist and Thorssen are moon-cattle hunters. The cattle are the source of the drug acret that is the only known cure:
Thorssen and I had the same type of job. We worked together on one of the most important industries there was. We were moon-cattle hunters in a period when the acret for which the moon-cattle were sought was the only thing that saved the life and intelligence of billions of the world’s population.
They find a tesseract, and in investigating it they fall through it and find themselves transported to another planet. Various adventures ensue, but the protagonist no longer has access to her supply of acret and she feels herself losing intelligence and physical stature. Eventually she decides to commit suicide:
My vision began to clear again and I could see Thorssen. He seemed to be angry with me. He wanted me to come over to him. I walked carefully to his side and stood peering up into his face. Thorssen was tall and wide and strong. I hated him to see me this way. I wanted to cover my face with my hands. I wanted to die. I was horrible.
The protagonist and Thorssen find another mechanism on the planet and the protagonist tries to kill herself by grabbing the power supply:
My left hand was on the helmet. My right hand reached up and touched the charged rod. A jolt of livid green flame lighted my way to darkness.
But:
My mind was clear, clearer than it had been since I could remember. The splitting ache was completely gone. My body was strangely relaxed, vital. I felt wonderful. I wasn’t a cretin! But why not? In fact, why wasn’t I dead? I had committed suicide, hadn’t I?
I opened my eyes wide and stared at Thorssen. I moved a little and he stopped instantly, smiled down at me. There was something about his smile that dazed me.
“Martha,” he said gently, “there’s nothing to be afraid of—not now, with what you have ahead of you.”
But I was frightened now. There was something to be frightened about, something changed. He was not looking at me as if I were a pitiable creature of a shunned race. I saw myself as a woman in his eyes, the woman I had never been for him before. I struggled to be put down. The instant I touched the red rock wall, I felt the difference. I was changed! I was long-limbed and slim with a new grace. Shakily, I touched my hair. It was luxuriant and silken. My skin was clear and smooth. Wordlessly he handed me a highly polished bit of metal he used for shaving.
The face was almost a mockery, it was so different. All my features were refined to the point of sheer exquisiteness. My eyes were wide, clear and a frightened green. My hair, dull before and lank, was a glory of shimmering gold-red. The trembling lips reflected were red and soft—vibrantly young. I was beautiful, more beautiful than any woman I had ever seen and envied.
Thorssen tells Martha that while she was unconscious one of the original inhabitants of the planet appeared to Thorssen and told him that by activating the machine Thorssen and the protagonist had freed them. This would be the wise old man you remember. He then opens a new tesseract so Martha and Thorssen can return home. Thorssen tells Martha:
“You can tell me now,” I said slowly, “why—why I’m alive. I tried to kill myself when I found I was becoming a cretin. How did I fail? And why am I not a cretin right now? And this miraculous change . . .
He took my hand in his and held it tight. “That wasn’t electricity, Martha. I don’t know what it was, but it seems to have been a sort of basic life-force. As a favor, my visitor gave me a slight dose of it—look!” He bared his shoulder. It was healed entirely. “Your cretinism was cured at the same time. Your thyroid has been regenerated. You’ll never need treatments again. In fact, with a dose of the force you had, I doubt you’ll ever need treatments for anything.”
